In Lindy Hop, the Roll-up is a medium-difficulty lift / aerial that relies a lot on timing, connection, and leverage.
It apparently also causes a lot of friction and / or bruising.
Apparently, the inner-biceps of the lead and the inner thighs of the follow are prime locations for this. The inner thighs of the follow are potentially bruised by the leads hands as he uses his upper body to basically make the follow do a somersault in mid-air, while the lead's biceps are apparently brutalized by the follow's funky looking belt, if she happens to be wearing one as the outer hips rotate around that area.
(Sorry, no inner-thigh picts.) :P
Of course, there's always the danger of being "coochie-punched in the throat" by your follow as well... but that's a completely different unsavory story.