I'm just a backyard mechanic, but here are my thoughts. You have 2 lines going into the the bottom tank on the radiator. One line goes into the tank from the transmission and the other line goes from the tank into the transmission. Fluid is constantly traveling through these lines to keep the tranny cool. Sounds to me like there is a leak inside the bottom tank that is allowing tranny fluid leak from tranny line inside the tank and getting into the coolant. If this is indeed the problem, you need a new radiator. I had a problem with the tranny fluid getting water in it. The tranny is a 727 tranny in a '73 Dodge Challenger. I had to put another tranny in it and to make sure it don't happen again, i put a tranny cooler n the car. So yes, I would say it could be fatal. I hope this info helps