If you butcher a humanlike corpse with a pawn that does not have a 'mood' need (Example: Alaestor's Robots++ (http://steamcommunity.com/sharedfiles/filedetails/?id=747645520) 'Kitchen Bot') the butcher job will throw an error, not complete (and restart again, to just cause another error again & again & again....)
System.NullReferenceException: Object reference not set to an instance of an object
(Triggered inside Corpse.<ButcherProducts>c__Iterator22C.MoveNext())
Since I'm currently writing a mod that mucks around in the butchering jobs, and thus am knee deep in (code) guts anyway I took a look and found the cause of the bug:
The Corpse.ButcherProducts Iterator does not check if the butchering pawn does actually have a non-null 'mood' property before trying to add a 'ThoughtDefOf.ButcheredHumanlikeCorpse' thought. Thus this instruction blows, and the method never completes.
The code does check all pawns who should get a 'ThoughtDefOf.KnowButcheredHumanlikeCorpse' debuff ;)
I've actually put together a bugfix mod that includes code (https://github.com/DoctorVanGogh/Rimworld-ButcherMoodFix/blob/master/Source/RimworldButcherMoodFix/Patches/Corpse_ButcherProducts.cs#L29-L45) which correctly catches this condition (needs a C# 6.0 capable compiler, otherwise you'll have to manually unfold all those '?.' null propagations).