3. Zach Hyman
If you had a player up for a new contract and he wanted to more than double his money, at the age of 30, and he wanted to sign for the maximum length, which would have run the contract into his late thirties, and if he had a history of playing poorly in the playoffs, two bad knees, a lengthy injury history and a play style that made it seem like he'd be injured more often than he'd play, and if that player had a career high of only 41 points, it would be a no-brainer to let him walk in free-agency.
Unless you are the Leafs, then that no-brainer would turn out to be the worst decision you ever made.
When the Oilers showed up to give Hyman more money and term than any reasonable person would think was smart, the Leafs were only too happy to drive him to the airport.
But like Nazem Kadri before him, Zach Hyman's departure would haunt the Leafs in almost unimaginable ways.
Hyman (who would have been happy to be a career fourth liner based on long odds of him even making the NHL when Kyle Dubas first acquired him) ascending to become a franchise player and one of the best players in the NHL wasn't even something considered to be in the realm of possibility.
But then again, this is the Leafs we are talking about, so maybe we should have known better. After Hyman left Toronto, he scored 27 goals and made the Leafs look pretty dumb.
The next year he scored 36 times and scored 83 points, which made them look even dumber. But last year was something no one could have imagined. Hyman scored 54 goals and another 16 in the playoffs for a total of 70 on the season, while his team came within one game of winning the Stanley Cup.
This doesn't happen. In the NHL, players just do not improve and have their best seasons at age 32. Unless, of course, they are smarting from rejection from the Toronto Maple Leafs, in which case it becomes almost routine.