The first thing that came to mind when I saw this thread was something that happened when I was waiting tables (you see many strange things waiting tables!
I used to work at Outback Steakhouse and one of our popular salad dressings is Mustard Vinaigrette (MV). This older gentleman ordered his salad with the MV, plus extra dressing on the side. After his salad was taken out, I swung by to make sure everything was fine. This guy had finished his salad and was sucking what was left of the dressing out of the bottom of his salad bowl with his straw!
It was pretty gross! I of course immediately tell the other servers and they all make a point to walk by and see it for themselves
FYI: If you are eating out and it seems like there are a lot of servers walking by your table all of sudden... its probably because you (or someone you are sitting with) are doing something odd and they want to see what's going on...