Bosses going Warwolf have been mentioned, but I still remember my first encounter with Clockwork Gears.
I was solo on my first defender and had just managed to put down a Prince by myself. My health bar was in the low yellow and my end bar wasn't doing too good either (pre-Stamina + Clock shocks), but he was down... and then... I distinctly remember thinking "OMG it had
kittens"
Then there was the time I was flying over Skyway, saw something odd, and went down for a closer look.
The "something odd" turned out to be a
very purple (in attire as well as level) Freakshow, who sent me to the hospital the quick way.
Honorable mention: not realizing for at least a couple of weeks that I could combine enhancements
without both of them needing to be already slotted on the character. That led to some...
interesting slotting choices and juggling, though I was too low level and short of slots for it to matter much before I found out otherwise.