Related to bug 24178, when you move a thread from FOO -> BAR you get a redirect on FOO, so your forum on FOO will be:
= My thread Moved to BAR blah blah
But and on BAR:
= My thread The original content
However, if you then decide to change your mind and move "My thread" back from BAR -> FOO you end up with this:
= My thread The original content = My thread Moved to BAR blah blah
I.e. there are now 2 threads under the same name, the now-broken redirect and the original.