Corpses of corpseless monsters
ShivanHunter has showed during TNNT 2018 that it was possible in the development version of NetHack 3.6.2 to encounter corpses of monsters that do not normally leave corpses. This occurs when a monster migrates to another floor, such as by falling down a hole or trap door, but there is no space on that floor for them to arrive. This also occurs in NetHack 3.6.1.
In the game where they found this out, they then proceeded to ascend with two tins of grid bug meat. The dumplog is here.
The bug has now been fixed in the NetHack 3.6.2 beta.