For me it was the Tavares signing. I'm not sure if it was this board or another forum in which I voiced my strong displeasure.
It's not that Tavares wasn't worth the contract or the risk - he was and it's only our hindsight that makes us think otherwise. We knew that the latter years of this contract would be suboptimal relative to his AAV. At the time, a 27 year old, hometown kid, signing for a discount with the upside of hitting 100+ points per season without having to give up any assets seemed like too good to be true.
The problem is that the signing made absolutely no sense given the timings and trajectory of the Leafs, especially with three young players due for raises. For someone who was seemingly a cap and statistical savant, Dubas shortened the Leafs' content window by a solid 5 years. If anything, it was irrational hubris ("we can and we will") that led to this lapse in judgement and screwed this team's cap structure for the next decade.