Found the trigger for a long-standing bug that would corrupt savegames beyond repair. It is caused by the player character hallucinating and the game making a fake item as an appearance for a tool. objnam.c creates such items whenever a hallucinated appearance is requested for any item, but when deleting them it fails to check whether the item has contents, and in the case of tools the contents may be eggs or other timered objects which aren't properly cleaned up. When that happens, saving the game will throw an obj_is_local panic and leave a broken savegame that cannot be recovered.
I've fixed this error in my own variant here: https://github.com/SLASHEM-Extended/SLASHEM-Extended/commit/90135789125d07207eea18d0bcbd54cfe6855760
Can be reproduced by the following steps: wish for a potion of hallucination, quaff it, wish for a couple tools (doesn't matter which ones), repeatedly open and close the inventory, and save the game. Eventually it will break. Can also use #timeout to check for egg timeouts being added, which are a sign of the bug striking.