Overlay unkillable

[Andrea Crotti]

Sometimes I do the followoing (more or less by mistake).
Go in an orgmode block, press C-c ' to edit in the right mode, then
change the mode (sometimes it makes sense).

After I do that pressing C-c ' doesn't go back correctly and more
importantly I can't kill that buffer anymore, because it's an overlay.

Anyway else had this problem?
Well it's not a big deal in theory since I should just remember to avoid
it, but it's strange to have a buffer around which is unkillable...