At 2:42PM EST I got this error spamming in the server console
[ERROR] lua/includes/modules/constraint.lua:177: Tried to use a NULL entity!
1. GetTable - [C]:-1
2. RemoveConstraints - lua/includes/modules/constraint.lua:177
3. unknown - ...p/entities/weapons/gmod_tool/stools/pnrp_powerlinker.lua:70
4. Do_Undo - lua/includes/modules/undo.lua:349
5. CC_UndoLast - lua/includes/modules/undo.lua:405
6. unknown - lua/includes/modules/undo.lua:416
7. unknown - lua/includes/modules/concommand.lua:69