Many younger fans may not be aware that Kenny Lofton got his start in baseball with the Astros when they selected him in the 17th round of the 1988 MLB Draft. It took Lofton a little while to get going, but he eventually started hitting and made his big league debut in 1991 and struggled in limited action with a .203/.253/.216 line. At 26 years old and low on the Astros' outfield depth chart, the Astros traded him to the Indians (now the Guardians) before the 1992 season.
After leaving Houston, Lofton made a point to make the Astros regret trading him as he became a defensive highlight reel and one of the most feared base stealers in the league during his career. He would lead the league the next five seasons in steals, make six All-Star games, and put together a borderline Hall of Fame worthy 17 season career.
We can't actually fault the Astros too much here. Lofton was already pretty old for a prospect who hadn't demonstrated the ability to hit big league pitching. Unfortunately, he figured things out in Cleveland and the Astros have to wonder what might have been.
Another later round pick that could have been a steal for the Astros if he hadn't gotten away is JD Martinez. The Astros picked Martinez in the 20th round of the 2009 draft out of Nova Southeastern University. Martinez would absolutely mash in the minor leagues before getting called up by the Astros in 2011. Martinez was only okay in his three seasons in Houston with a combined .251/.300/.387 line with 24 home runs in 975 plate appearances. Despite making some swing changes that seemed promising, the Astros released Martinez before the 2014 season.
This was a lesson in not underestimating a player's ability to improve. Martinez would sign a minor league deal with the Detroit Tigers a couple of days later and got called back up to the big leagues that April. Martinez would go on to annihilate baseballs for Detroit, Arizona, and Boston and is still doing good work at the plate for the Dodgers to this day.
It sure seems like the Astros should have paid attention to the adjustments that Martinez made instead of just assuming he couldn't fix his issues.