-Spending 2 hrs getting wrangled, riot controlled, you name it, into a ql120 nova when my soldier was level 50. Going on a mission in somewhere remote (it's been a while) loving the new rifle, firing away and suddenly punching. No ammo, didn't make it out, died, lost a lot of xp, was fed up and angry :\
-Rooting my lvl8 NT when running away from a mob (I guess we all did this at one point or another)
-Going into the Den thinking you've saved that 50% xp you need to level, then getting raped by two den loot controllers realising you ...didn't.
-Charming a leet and being attacked by guards, loosing pretty much everything -30xp I needed to level. (funcom! baaaad)
-Going for a piss thinking your player is in a safe spot in the mission, coming back staring at reclaim
-Putting the heal pet on the mob and not realising until you have 10% hp left (it is actually possible, don't ask me why).
-Flying around with the newly aquired yalm jumping off buildings, then trying to stand on top of the spear that the whompas have, finding out you can't, landing on the gate, then jumping off and dying after loosing 70 hp.
And a lot of other stuff.