I have been pulled at the finish of a 50 twice in the last 3 years. At
one, it was poor management on my part. It was a cold, rainy day & I
allowed the horse to cramp while standing in line for P&R's. At the
other, my mare cut her heel on a rock in the last mile to the finish
line. When the adrenaline ran out, she was very sore on it/ Grade III
lame.
It DOES happen, and it was a deserved pull each time. (That didn't
lessen the disappointment any, though.)