Is it possible for a roller coaster to go in reverse (unintentionally)?

It normally can't roll back on lift hill (anti rollback is the thing making the clicking noise as you go up the lift hill.The track is split into zones so 2 coasters can't be in the same zone so if the first train doesn't clear zone(hill) it can't rollback into another train.I have a few hundred lap on different coaster and have never had a rollback.