The draft can be one of the most exciting days of the NFL season, particularly when you're a fan of the Cleveland Browns. (Sorry, Browns fans.) Players enter the NFL with so much promise and potential. And many of them will go on to enjoy successful careers in the league.

But many do not. And that's because the NFL draft is a game of roulette - teams never know what kind of player they are going to get until they see what he's like in an NFL uniform and professional environment.

These are the players whose careers went dramatically wrong. They were among the top selections in their draft, and yet they emerged as either non-factors or, even worse, consistent detractors for teams.

Here are the 13 biggest draft busts and disappointments in NFL history.

13. Trent Richardson, RB, Alabama

Draft details: Cleveland Browns. 2012. Third overall.

There was uproar when the Browns decided to flip Richardson to the Indianapolis Colts for a first-round pick. How could they? Then folks studied Richardson's stats and found that the Browns had fleeced the Colts. Richardson may have had 950 yards and 11 touchdowns in his rookie season, but he needed an absurd 267 carries to do it. On his career in the NFL, he averaged 3.3 yards per carry. (He was even worse in the AFL with 2.9 yards per carry.) With the Colts, Richardson had 458 rushing yards, 265 receiving yards and four total touchdowns in 2013 and had 519 rushing yards, 229 reviving yards and there total touchdowns in 2014.

The Browns knew he wasn't good, so they were happy to bail on him. The Colts must have felt like they needed to justify the trade, which resulted in his large workload. As a result Richardson has large sample size of oppressively bad performances.

12. Courtney Brown, DE, Penn State

Draft details: Browns. 2000. First overall.

Brown's biggest problem was injuries. After a promising rookie season, when he had 69 tackles and 4.5 sacks, Brown suffered an injury in the fifth game of the 2001 season. He missed the rest of the year. He then missed five games in the following season and three games in his final year in Cleveland. He finished his career with the Denver Broncos in 2005 when he had 24 tackles and two sacks.

When his career ended after six seasons, he had 19 sacks and 155 tackles, a pair of brutally small sums for a former No. 1 overall pick.

11. Andre Ware, QB, Houston

Draft details: Detroit Lions. 1990. Seventh overall.

He played in just 14 games and started six. In those games, he had 1,112 passing yards, five passing touchdowns and eight interceptions. His completion percentage was 51.6. The Lions had some success while he was on the team, making the NFC championship after the 1991 seasons. But Ware was watching from the sideline while veteran Erik Kramer led the offense.

There's not much to say about Ware, because he did so little in the league. He couldn't win the starting job in Detroit, so they cut him after the 1993 season. He had brief stints with the Oakland Raiders and Jacksonville Jaguars, but never played a game for either team, with both organizations cutting Ware.

10. Brian Bosworth, LB, Oklahoma

Draft details: Seattle Seahawks. First round. 1987. Supplementary draft.

The Boz had a fairly epic start to his career. He kicked things off by sending letters to inform NFL teams that he would not report to training camp if they drafted him. His intent was to land with the Oakland Raiders. It didn't work.

The Seahawks drafted him and signed him for a record-breaking deal: 10 years, $11 million. It was the biggest contract in team history and the biggest rookie contract in the league's history. He played just three seasons, and retired due to a shoulder injury after playing 24 games.

He was nothing but hype and a bad haircut.

9. Blair Thomas, RB, Penn State

Draft details: New York Jets. 1990. Second overall.

In his rookie season, he didn't look terrible. He had 123 carries for 620 yards and one touchdown at five yards per carry. His second season was even more productive, but it was less efficient. He finished with 728 yards and three touchdowns at 3.9 yards per carry and added 30 receptions for 195 yards. Those were his best years, which isn't saying much.

He never had more than 500 yards in any of the following seasons, largely due to injury issues. The Jets parted ways with him after the 1993 season, and he couldn't find any success with the Dallas Cowboys, New England Patriots or Carolina Panthers. The three teams gave him a whirl, but found his production wanting.

8. Tim Couch, QB, Kentucky

Draft details: Cleveland Browns. 1999. First overall.

He was bad for all of his career. And yet he was just good enough that the Cleveland Browns stuck with him for five seasons. He went 22-37 as a starter with 11,131 yards, 64 touchdowns and 67 interceptions. His interceptions percentage was 3.9, and his yards per attempt were 6.5. He persisted long enough - with occasionally solid performances - before the Browns cut him after missing 18 games in five years, largely due to injury.

7. Rick Mirer, QB

Draft details: Seattle Seahawks. 1993. Second overall.

He managed to have a long career, even if it wasn't a successful one. But the large sample size (80 games) only magnified his inability to improve. He played for five different teams (and spent time on seven teams), and found little success on each roster. Worst of all, Mirer completed 51.5% of his passes for 420 yards, zero touchdowns and six interceptions in seven games play with the Chicago Bears in 1997.

Mirer's greatest success was probably his staying power in the league, as he found himself a longstanding backup for a handful of teams.

6. Tony Mandarich, OT, Michigan State

Draft details: Green Bay Packers. 1989. Second overall.

He actually played in 86 games and started 63, which doesn't that bad. But it is. We promise.

His first stint in the NFL was disastrous. He played for the Packers for three seasons, before they cut him. It didn't help that he was a total jerk during his Green Bay tenure. It probably also didn't help that he was a heavy steroid user. After leaving the Packers, he took five years away from football, in part to address drug and alcohol issues in rehab. Somehow, he made a return with the Indianapolis Colts, where he was as serviceable starter by the end of his three-year stint.

5. Heath Shuler, QB

Draft details: Washington Redskins. 1994. Third overall.

He's one of the many quarterbacks on this list on the sub-50% club, ending his career with a completion percentage of 49.2. His touchdown-interception ratio was equality atrocious, with 15 scores and 33 picks. In 29 appearances, he mustered 127.3 yards per game and his record as a starter was 8-14. Amid Shuler's lack of success - including a 1994 appearance when he there five interceptions - Gus Frerotte emerged as a legitimate starter, and by Shuler's third season, Frerotte won the job away from the former third-overall pick. Shuler concluded his career with a passer rating was of 54.3.

4. JaMarcus Russell, QB, LSU

Draft details: Oakland Raiders. 2007. First overall.

Russell had one season in 2008 when he wasn't an utter failure. That season, he went 5-10 while completing 53.8 percent of his passes for 2,423 yards, 13 touchdowns and eight interceptions. Without a doubt, it's a bad season. But it wasn't a hopeless season like some of the other years. His interception percentage was down to 2.2. His yards per attempt were up to 6.6.

But Russell, who was using prescribed and non-prescribed Codeine syrup during his career, had a brutal pair of seasons in 2007 and 2009. He finished his career with a 7-18 record while throwing for 4,084 yards, 18 touchdowns and 23 interceptions. His failures have been magnified because of his massive contract ($61 million total with $32 million guaranteed), but he's not the worst pick in history.

3. Akili Smith, QB, Oregon

Draft details: Cincinatti Bengals. 1999. Third overall.

He concluded his career with a 46.6 completion percentage. That's staggeringly bad, even when considering he completed just 56.6% in college (which begs the question of why the Bengals picked him third). He played in 22 games and was 3-14 as a starter. Over his final two seasons in Cincinnati, he played in just three games while backing up an atrociously bad Jon Kitna (12 touchdowns, 22 interceptions in 2001). Still, Smith was in his offensive coordinator's doghouse for a lack of work ethic.

That may have been why he never stuck elsewhere in the NFL. Once Cincy cut him, he got cut by the Packers and Buccaneers and resorted to play in NFL Europe and the CFL.

2. Charles Rogers, WR, Michigan State

Draft details: Detroit Lions. 2003. Second overall.

His lack of production is staggering. He lasted just three seasons. He played in just 15 games. He logged 22 receptions for 243 yards and four touchdowns. His first two seasons were shortened by two separate clavicle injuries. A substance-abuse suspension abbreviated his third season. After that, the Lions were sick of him, and he never held another NFL job.

He is truly a marvel of missed – and neglected – opportunity.

1. Ryan Leaf, QB, Washington State

Draft details: San Diego Chargers. 1998. Second overall.

He went 4-17 as a starter while throwing for 14 touchdowns and 36 interceptions. He never had more touchdowns than interceptions in a season. He never completed more than 51.1 percent of his passes in a season. He never won more than three games in a season. His interception percentage was an absurd 5.5 during his career. In his final season, his sack percentage was 12 percent. He did absolutely nothing well as a quarterback.

We now know that Leaf was troubled with drug and addiction issues, which surely impacted his ability to perform on the football field. And it's great that he's turned his life around. Still, he is easily the worst selection ever made in the history of the NFL draft.