Not encountered before today but I made two consecutive posts to a thread, then wished to edit the second of them. On clicking 'Edit' in the second post......... the first was opened for editing.
Thinking that I may have made a silly mistake, I cancelled and repeated the process - with the same result!
I then asked another moderator to try the same but all worked as it should for him.
This has only occurred once in over 60 posts (we're still at the testing stage)