I have been the F/O for two rejected T/Os. The first was a non-event and we just slowed down and taxied back. A flock of birds decided to fly around the runway as we were approaching 60 knots. No biggie.

The second was a bit bigger and we rejected at about 100 KIAS at LGA on RWY 4. That was a bit more interesting, but we still were able to stop and exit the runway before the intersection of 13/31. The passengers were not freaked out about it, but were p/o'd because they might miss their connections at IAD!