Playing Battle Arena at all. The majority of my deaths in that just shouldn't be possible, like opponents walking into a defensive Nagrants and killing me while standing inside it because the server hears they killed me before it hears they committed suicide to Nagrants.
Leaving that aside though, the weirdest one was when my controller failed during the Episode 4 LQ. It did so in a way that also disabled most keyboard input excepting like movement and menus. I ended up resetting my entire UI during the LQ to try and fix the mystery input problem (unwittingly turning on voice chat in the process to the amusement of the party - why is voice chat on by default btw?) before unplugging the controller restored keyboard controls and let me move from where I'd been revived and catch up to the others who'd almost advanced two areas by that point. We still won!
The dumbest was fat-fingering a Shifta subpalette input at the start of the second area of Trick or Treat, causing me to Ilzonde into exploding barrels.