Why do combines (e.g. Rate Meat + Loaf of Bread in Spit) sometimes stop working?

[Wizardling]

Old or new 1999 only bug? Solution other than logging off and back in?

[Wizardling]

Update: logged back in this time to find an item - a clump of meat - inside my spit that had previously been invisible (plus the spit still gave the "You must place items inside" error when empty in last session). Weirder is I never put it in there.