If the compression and rebound damping is too soft, the bike will pogo up and down a couple or a few times. If the rebound damping is too soft, you can get a nice compression bounce, but the fork or shock will over-extend and again you get more than one bounce. If the rebound damping is too hard, it won't allow the suspension to return quickly to the rest position. In the extreme case, the suspension would be completely collapsed and you would be riding a hard-tail Harley. Once again, if the wheels can't return to the road quickly enough, the wheel will skip off bumps again.
Intermediate symptoms are loss of ground clearance.
As you can see, you can't just tweak the settings and go on your merry way. You'll have to mess with them a few times until you get it right. Ideally, you have to reset all of the settings. Once you get used to the characteristics of the dampings, you can make fine adjustments over the course of a road trip. The best thing is to find a stretch of road with a couple of curves with bumps in it.