Bending the knee backwards is a regular one and causes a condition called Bockers Knee
Not serious but a little irritating
Once you build up the muscles it should pass also wearing something like the Pit Dirt knee pads helps as it takes some of the strain off of the knee joint
When I used the knee bars I used to adjust them a little lower so the pads located on the bar and moved the pressure points onto my shins this helped with the pressure on my knees