Measures are too easily corruptible?

I really don't know how this happened, and unfortunately I do not know the steps to reproduce this bug. But I'll write down what I remember.

1) Create a new solo piano score in 4/4 time
2) Write in two voices for two measures
3) I think I deleted one of the rests in the bottom voice via backspace?
4) Ctrl+X (cut) both measures
5) Try replacing the 4/4 time sig with a 12/8 time sig
6) The pop-up in the image attachment below appears
7) ????????????????????????????????
8) Profit? (The issue miraculously fixed itself after I left for lunch)

Again, any attempts to reproduce this bug have been futile: I couldn't get the measures to "corrupt" so to speak. Do let me know of any reliable ways to get this to trigger, though.