I thought a post was in the wrong forum so I wrote a reply to it saying that it was in the wrong forum and that I was moving it.
I then moved it to a different forum.
When checking it in that forum I realised that hidden (to me) in the text was the indication that it had been in the right forum in the first place, so (still in the second forum) I deleted my post. That delete worked.
I then moved the thread (now consisting of one post and a deleted post) back to the original forum.
In *that* forum my post was no longer deleted (although the Delete comment was visible) and I had to delete it again.