2010, Finals Game 7
2018 ECF, Game 7
2020 Bubble vs. Miami
2022 Finals after Game 3
Honestly I'd even say most of last year's postseason. I mean sure, they beat ATL and PHI but those series went way longer than they should have and the C's looked meh. Then falling behind 3-0 to start the ECF even with HCA.
If you want a single answer though, I'd probably lean last year's ECF Game 7. Literally it was there for them to make history at home to reach the Finals, and they yet again choked badly. Sums it up for the organization really, they've been close many times and just can't get over that hump for Banner 18