I vaguely remember some... left over thing being shoved into the back of the fridge. Several months later, there was black slime dripping down the back of the fridge, accompanied by an incredibly offensive odor. My parents practically power-washed the fridge after that.
The grossest thing in my fridge by far, though, were ants. Yes, I had ants in my fridge. Imagine going in there for some sliced turkey, but when you open that package up, that's right, there are ant corpses in there. At least they were fresh ant corpses, though!
Also, although not in the fridge, when I was about six I put a pear core into my jacket pocket. I forgot about it for a week, and, after putting my hand in my pocket, I got a nasty surprise. Of course, with my six-year-old logic, I decided to never wear the coat again, not tell anyone, and leave the fucking pear inside. About a year later, forgetting about the whole pear incident, I felt in my pocket to find... black dust and weird fibrous stuff. I didn't realize until later that what I found was the decomposed pear core from such a long time ago.