While I don't think that "someone would have noticed" is always a fallacy, I do think that we humans tend to underestimate the chance of some obvious fact going unnoticed by a large group for a prolonged period.

At a computer vision conference last year, the best paper award went to some researchers that discovered an astonishing yet simple statistic of natural images, which surprised me at first because I thought all the simple, low level, easily accessible discoveries in computer vision had long since been discovered.

A different example- one of the most successful techniques in computer vision of the past decade has been graph cuts, where you formulate an optimization problem as a max flow problem in a graph. The first paper on graph cuts was published in 1991 iirc, but it was ignored and it wasn't until 2000 that people went back to it, whereupon several of the field's key problems were immediately solved!