You are on the right lines in mounting a defence by referring to the previous incident you witnessed. As others have said, eating any stock in a restaurant or supermarket is typically treated as theft, whether it was a bruised banana or dented can of coke, the 'zero tolerance' approach is widely practiced. This makes sense because no-one wants to have debates about what was unfit for sale or not, and even if already deemed 'waste' that doesn't necessarily mean you are entitled to eat it. The big BUT is that it all depends on how clear any rules on this are, AND how they are carried out. So, if there are huge signs in all the staff areas saying "EATING STOCK IS THEFT - YOU MAY BE DISMISSED" then you have a pretty hopeless case. However, if nothing is explicit, and your boss tolerates or even accepts others eating/drinking stock, then you have a very strong case to challenge any formal disciplinary action. Your best argument is to say you were only doing what had been tolerated and couldn't therefore have known it would result in formal action. If the worst happens, and you are dismissed, you can only pursue it at an employment tribunal if you have more than 12 months service. Good luck.