Do you remember the last steps you made before the error message came?

I was walking around the SMC. I don't think I did anything out-of-the-ordinary. I was just coming back with the blanket and stepped on the stairs to descend to UD:1 (I don't believe I pressed descend before it crashed). I don't remember seeing a stone snake or any of the other monsters it listed being in the area.

The same thing happened to me, but it did not crash.

I was in the SMC (ratling duelist), I ascended to Level 5, then I grabbed the blanket and descended to UD:1.

There was a serious of ~many~ messages, maybe 50 total that read:

Putting a monster (ochre jelly, 203) at a blocked place (72, 11) [5] {8/2}
*** I'm trying to fix the problem by deleting the monster

This ran many many times, with different monsters (most of them seriously out of depth for Lvl: 5, UD:1, I think a vampire was named)

Then, there was a series of this message:

Stacking monsters (a troll/34 {1073741824, 0} [-1,-1] with a stone snake/171 [72,11]) at (72, 11) is not allowed (L: 8. 2; LI: -1).

This ran many times, maybe 50 total. Always [72, 11], but different monsters (ice vortex, green hag.)

It finally went through (!) and then: "You feel a surge of pulsing power!" which I am excited to now investigate.

Oh, I might add that a boss monster was generated while I was on the SMC but I didn't see it.

The problematic UD:1 had an unconnected 5x5 room with Bugbiter in it, FYI. I didn't get to see what was in it (I died on UD:1 shortly after)

Already found the problem, just a simple copy&paste error. This "unconnected room" is a new feature...

So I had a crash and missed out on an early artifact? That's disappointing :(