The two big ones that come to mind for me as a Habs fan are the Drouin and Gomez trades. Both times the Habs attempted to trade a defenseman for a talented forward to rectify our massive lack of scoring and both times they ended up being a failure. McDonagh as traded for Gomez and he ended up being a top pair defenseman through his career. It looks like it will be pretty much the same thing with Sergachev, who was traded for Drouin. Gomez had a terrible contract at the time of the trade and I remember people talking about how he was visibly declining. McDonagh wasn't really lighting the world on fire in college but ended up being pretty good. When Drouin first came to the habs he wasn't quite a star or anything but he was a talented top 6 forward that was asked to play C when he shouldn't have. He ended up dealing with a number of injuries and mental health issues and just steadily declined.