Josh Donaldson, 2018
Right before the beginning of the 2018 season, the Cardinals reportedly offered then-prospect Jack Flaherty to the Toronto Blue Jays for a one-year rental of Josh Donaldson.
The Blue Jays said no, which turned out to be a very good thing for the Cardinals. Donaldson played in just 52 games in 2018, posting a .801 OPS, while Flaherty was a stud for the St. Louis rotation in 2018-2019.
Where things could have really gone south for the Cardinals is that they likely would have tried to extend him if they were giving up that kind of asset to get him. So not only would they have dealt Flaherty, but they may have been stuck with Donaldson long-term, instead of grabbing Nolan Arenado from the Rockies.