I was able to reproduce it today a few times, until i was not... :-) Switched tab, edited a file, saved, switched tab back and #0 was there. Did the same. Was there again. Then i tried the same, just without saving. Was there again. And then tried both of the above but it did not happen again :-).
But yes, it seems to be something with switching the tabs. -- <https://forum.pspad.com/read.php?2,75694,75935> PSPad freeware editor https://www.pspad.com