When upgrading a room to v12, we accidentally ended up mutating the
content of the old power level. Since we cache events, this meant any
future usage of the old power level event would see the wrong content
(until it dropped from the cache).
This meant that the creator of the new room would not be able to perform
admin actions on the old room. Any federation requests for the event
would fail the hash checks, since the content had been changed.
All in all, quite a nasty bug.
Erik Johnston
ae242fd11d