Reported for nethack.alt.org where impossible events in to-be-3.6.1
trigger a panic instead of just a warning, being polymorphed into a
vampire and draining something to 0 HP (rather than killing it with
the bite attack) didn't properly kill off the victim unless it was
also being drained from level 0 to level -1, resulting in impossible
"dmonsfree: 1 removed doesn't match 0 pending". If the hero got
another move before dead monsters were purged, applying a stethscope
to the victim could trigger an actual crash rather than an impossible
escalated to a panic. Other actions such as attacking again probably
could too.
A followup included a diagnosis and one-line patch. I expanded the
adjacent comment when manually putting the patch into place.
263bd05fa0