I've always noticed speed bumps before I hit 'em. But once I hit a ditch a bit too fast.
I was driving a pick-up in the woods and there was a small clearing that had tall grass groving on it. Accelerated and shifted to 2 low and... hit my head on the roof. It hurt.
The small ditch was so well camouflaged that I almost stepped into it when I looked around the truck for damage. There was non by the way.