I used to do one every time I was out doing a days worth of shops, there was always one along the route and I usually got some kind of bonus.
I had done one that got a semi not so good review. It wasn't horrible but not great either. This one was in a major shopping center so it would be completely normal for them to have repeat customers and even very regular customers.
I walked back into the shop over a month later and as I walk in the door the manager literally snaps her neck doing a double take and talks quickly and quietly to the rest of the crew. As I approach I hear the fry guy say, "but we don't get shopped on Thursday".
As he's shaking the fries he's looking at me.
I was clearly busted.
My point is, the only way they could have recognized me was by matching the time to the tape and having my picture.
Anyway, I didn't get blocked from them right away a few months later they disappeared. I figured I hit some kind of maximum. They just came back last month...it was about a 5 month layoff.