Well I have actually done this a few times (more then I would like to admit). We were down town on saturday at the food court getting lunch. I went and ordered mine gave the person a $20 note and then taken my food and sat down and ate it. It wasn't until DH asked for the change so he could get his lunch that I realised i hadn't stayed to get any. Lucky when I went back the man serving me remembered me and had it ready waiting.

I have done this ssooo many times, even asking to get extra money out on eftpos and then forgetting to take the money - just walking off without it.