People use 'layers' in digital art. The layers separate out their work so it doesn't become mixed up, however, sometimes people change the layer they're using accidentally and draw on the wrong one. It's a common mistake, but people find it very frustrating!
If I'm just starting it, I'll just use the Undo button, make a new layer, and keep going from there. If I finish it before realizing, I usually just go "To heck with it", leave it as it is, and make a mental note to put it in a separate layer.
Haha! I'm not alone in the world!! I thought I was the only one who did this, apparently I was wrong. I also use the wrong layer even though I have them named. I have to keep all of the other layers locked except the one that I'm working on.