Zack Wheeler, 2019
Approaching the trade deadline in 2019, the St. Louis Cardinals had the opportunity to add a very good starter in the form of Zack Wheeler, but the Mets were asking for one of Harrison Bader or Tyler O'Neill, and the club decided to pass on the move.
Wheeler ended up not being dealt, but was elite down the stretch, posting a 2.83 ERA during the second half, netting him a big contract from the Phillies in the offseason.
There's no guarantee the Cardinals could have brought him back in free agency, but the club does have a good track record of retaining guys when they trade for them. Since signing his new deal, he has gone 33-21 with a 2.91 ERA in 482.1 innings, finishing second in Cy Young voting in 2021.
I know Bader was a fan favorite, but it sounds like he could have easily brought an ace to St. Louis, so I'm shocked they passed on that.