My daughter does the same thing (peeing that is), and fibs about it. I was relieved to find out that it is normal for the age to have slip ups.
If it's a constant thing, try getting a pee alarm. I got one from the bedwetting store. It's more for night time issues. They also have medication.
They do get caught up in doing whatever, but if she has other issues, like ADD, it is a real struggle to pay attention. At least you know she won't go to college this way! Be careful what you do with her mind though - if she's told that she's lazy, and the problem is really outside of her control, that could really mess things up. Have her clean herself up, and do your best to not gag, and treat her like she's capable. Shame comes on it's own and doesn't need a fanfare. She'll love you for understanding, and even helping.