My favorite example to trot out of how C behavior can lead to security problems is a Cap'n'Proto problem a couple years back, reported here.[1] While not entirely the same, there are some some similarities. The article submitted in that discussion does a good job explaining the problem, and the discussion covers even more possible issues. It pays to be very careful how these checks are done in C.

1: https://news.ycombinator.com/item?id=14163111