(I've never seen one, but I do know somebody who caught a Catharus thrush which fit all the biometrics (various measurements, like wing and tail) of Bicknell's, but otherwise looked like a Gray-cheeked. The problem was, he caught it in Toronto, which is nowhere near where Bicknell's should be (not that birds pay a lot of attention to what the field guides tell them). I don't know what he called it, in the end.)
Peter