I was working on a component character and in the composite was pasting base + accent from one master to the next. Going back to the accent I tried to copy the anchor from one master to another, because the anchor was missing from some masters. I must have failed to actually copy that anchor, though, e.g. mis-clicked. When I then hit cmd+paste Glyphs seems to have pasted the previously selected base + accent components — now within that component itself, thus sending the program down into eternal recursion with only force-quitting as a remedy.
This should probably be an impossible action, even if performed only by accident.