Low Branches
This is the most obvious hazard, but when you are focused on your mirrors and your backup buddy, low branches are easily forgotten.
Curbs or Large Rocks
This literally happened to us yesterday. We backed up into a beautifully paved rv spot and put the automatic levelling jacks down. Later as I was poking around in my storage compartment I saw that the jacks were an inch away from the curb. Had I backed up one more inch I would have done some terrible damage to the jacks. The same hazard exists with medium and large rocks, clear the pad before backing in to avoid setting your jacks down on a potential jack bender.
Steep Slopes
It's usually the last camp site left in the park and with good reason. Setting up on a steep slope can be hazardous, a hassle or at least uncomfortable. If you park on blocks to help level out, your jacks are still going to be set at an angle to the ground. This could result in them bending or buckling from sheer, lateral force. If you must park on a steep slope, don't use your jacks. It may be a bit uncomfortable but at least you won't have a repair bill to accompany a poor nights sleep.
Structural Overhangs
So we are aware of branches now, but sometimes we neglect the obvious. Structures beside the camp pad can be a costly hazard. Take care to notice these things prior to backing in, because you may only see the support posts in your mirror. The overhang itself may extend a couple of feet beyond the posts so even if you don't back in to the hazard, your slide may be in danger when extended.
Potholes
These nasty little passages to the underworld can cause the same type of damage as rocks or curbs while setting down jacks. If your jack is half in the hole, chances are good that something is going to bend. Have your backup buddy check that the jacks are not going to fall into this trap before extending them fully.