YOU DID NOT process as carefully as you thought or the print got contaminated somewhere after drying.
Tong for each solution and rinse them often. Give full continuous agitation. Wash your drying racks. Keep prints separated in the wash. Do not overuse solutions.
Ronald is most likely correct.. but I had another thought; how old is your paper? I've used REALLY old paper before that a friend of mine was going to throw away.. and I got some random fogged/dark splotches after developing..
It's very old mitsubishi paper. I have no idea how old, but I would guess at least ten years. I've gotten similar-looking spots on fresh papers, but they were much browner, and usually on poorly-processed workprints.