I've never had that happen before. If it didn't smell like mold though, you're probably fine. Did you get a chance to closely inspect the oranges after you'd racked the mead out from under them? Did the green/black rub off or anything? (sorry, due to my lab-rat nature, my tendency is to poke at anything I don't understand)
And oxidation is a primarily chemical process, it doesn't spontaneously breed bacteria.
If I have fruit that's sticking up through the surface of my JAO I do break the warranty and swirl it every few days just to make sure nothing funky happens.
"The main ingredient needed is 'time' followed closely by 'patience'." - The Bishop 2013
"Good grief! If someone wanted to murder you, all they would have to do is ship you a 55 gallon barrel of honey and watch you document working yourself to death!" - Vance G
"When you consider that laziness and procrastination are the fundamentals of great mead, it is a miracle that the mazer cup happens." Medsen Fey, 2014
"Sure it can be done. I've never heard of it, but I do things I've never heard if all the time. That is the beauty of being a brewer!" - Loveofrose, 2014