I learned that this mostly happens when you (accidently or whatever) delete an item with something masked to it. First you won’t see a difference, but when you hit “undo” some time later the machine seems to be confused – it doesn’t know what to to with the masked item without the other item it was masked to.
Only things that helps is being very very careful and also to save your work very very often. When deleting objects always check first if you have chosen the right item or if there is someting masked to it.
It it happens nevertheless loading the last save won’t help as JR said, unless you close the window/tab/browser with HM and start it completey new. When you load one of your previous saves now it will work – at least most of the time. It’s possible that you have to go several saves back until you get the right one since you don’t know when the item was lost.
Hope I’ve expressed myself correctly and you know what I mean … 
