[5.0] Hint: Lock your chests!
I just started my first session with 5.0.
A few levels in i began filling a chest with stuff i couldn't use immediately, as usual. Unknown potions and scrolls, surplus wands etc., the usual.
I went a few levels down, gathered some stuff and wanted to return to my chest, only to see that it was gone. Gave me a mini heart attack! I suspected a gelatinous cube, but in the first levels? Maybe from a poly trap? Several turns later i saw an "n" and read the message (from memory) "You see a water nymph rummaging through a chest". Sure enough it then proceeded to take stuff out and peppered me with all the potions and wands inside.
I survived and got most of my stuff back, but it was close.
So, always lock your chests and hope that you don't encounter monsters with a key! Scare monster and Elbereth are your friends, too.
Note: Yes, i read the changelog and knew that such a thing could happen. But after playing nethack for so long, much is muscle memory by now and i'm kind of set in my ways.
Now i'm sitting in dlvl 25, 70.000 turns in and have no means of levitating or flying. No boots, ring, amulet, spell or potion whatsoever. None! I could poly into a flying creature or dig through the Medusa lvl, come back from below and lvl teleport back to stash. I have the means for both, but i don't like either. Or i could polypile all the armor in Ludios and hope for boots. I haven't decided yet.