The most common one I have seen was taking french fries that had been sitting around for a couple of hours, dipping them in the fryer to heat them back up, and then serving them.
The worst was a boss with no sense whatsoever. He went into the dish room and started digging around in the trash and noticed a lot unopened coffee creamers, which must cost all of 2 cents each. He dug around for an hour and managed to find 100 of them (that’s a whole $2.00). He bitched about throwing money away and said to reuse these things. Well, since they had come out of the trash they were covered in crud. So his bright idea was to run them through the dishwasher! Then. mix them back in with the unused ones! As you can imagine, customers started complaining that the cream was spoiled and ruined their coffee. So he ended up giving away free coffee the rest of that night. What a DMF.