Undo failure?

I've noticed a few times in #3512 that when I try to undo a change, I sometimes get an Undo message box, but pressing the button to confirm the undo has no effect - i.e. the message box goes away, but the code is unchanged. Luckily, the Local History allows you to recover the original code.

Unfortunately I don't have details of the exact circumstances when I get the Undo message box, but I do know that my heart sinks when I see it...