The first seeds I germinated were Sun Gold and Brandywine OTV. I know I did not mix them up when sowing. They were planted in separate 18 cell flats so they couldn't have moved from one flat to another by themselves.

Of course it is possible that the wrong seed got in the OTV packet (seeds from TGS). The other possibility I thought of is that this seed is from a cross between OTV and another tomato that was growing near it. I wish I had more space for tomatoes, but I don't. I am curious to see what it is though. How likely is it to be a cross? I know this is difficult to answer without knowing the conditions in which the plant was grown, but in general, how rare is it for a cross to occur unaided by humans?

I am going to let it grow for a bit to make sure it isn't Sun Gold, but I would have no clue as to how that happened. The only thing I can think of is that we have lizards and toads that get on the back porch where I keep the seed flats and maybe they got in one flat, got a seed on their foot and then walked to the other flat and transferred it that way.