A gal in front of me just tried to pass an obviously counterfeit bill

I was at the Shell station and this woman threw a few candy bars on the counter and a big fountain drink and pulled out a fake looking $20. When the woman behind the counter told her, “we can’t take that.” The customer said,” oh, I must have grabbed the wrong one.” ( does everyone drive around with fake $20’s?)
She left the store and drove off, leaving her candy and drink on the counter.
After she was gone, the lady behind the counter said,” did you see what she tried to do?”
What possible outcome did the counterfeiter expect?!