Baggage you brought with you from other games
Personally?
I didn't realise flags like slicing, chopping, etc. and 'vorpal' were the same thing.
I expect gold to have weight, have been known to drop it to try and lessen my encumbrance.
When on CAO, I see swarms of killer bees and think they're kobolds and get poisoned to death; I think mushrooms are gelatinous cubes, and charge across rooms to attack them.
I'm still in the mindset that if a monster becomes hasted, or invisible, it will stay that way forever. It's always a shock to see one reappear out of nowhere.
I have a nagging feeling that too many mutations will turn me into a blob of primal chaos.
On the flipside, if I play any RL other than Crawl nowadays, I keep wondering why the autoexplore function is broken...