The same (?) problem was reported in Moved topics disappearing?.
You should see
Moving topics is for sure one action that can trigger it.
I can reply to an OP that the topic will be moved and next move it. After the move, the topic sometimes becomes "corrupt".
If I change the sequence to move followed by reply and it happens, I can't reply because the topic was (temporarily) hidden.
If there are already replies, the opening post disappears and the replies stay. Refreshing the page results in the OOPS page.