Mine has to be from 7 years ago (when I was 12 and only just starting out) when I was playing my first 1500 point game of space marines. I wanted to field Marneus Calgar with his command squad so bad but I didn't know how to play them, so I just set them up by themselves in my deployment zone with the intention of running them across the board into combat.
This was also the first time I'd ever seen Tau, let alone faced them and was promptly shocked at their firepower. However, this didn't stop my chapter master from making it across the board and wiping out a unit of broadsides and two full teams of fire warriors before finally being brought down! He was an absolute tank that game haha