Reported directly to devteam: monster vs monster location swapping
didn't handle single-segment long worms properly. Multi-segment
worms are disallowed but a worm with no visible segments (which
actually has 1 segment at the head's location) are allowed and the
segment wasn't being moved with the core monster and could trigger
warnings if sanity checking is enabled. The next time that the
worm moved, it got itself back in synch.
I couldn't reproduce the warning but mdisplacem() clearly assumed
that a long worm reporting 0 segments didn't have any so wasn't
attempting to handle the hidden one.
c3e5aaf8ba