1) If your skirt is short and denim and you sit down, there is a good chance the skirt will not fall down by itself like normal skirts do when you stand up. I recently witnessed this at a restaurant. A girl in her early 20s flashed the whole restaurant when her itsy-bitsy skirt remained at her hips when she stood to leave. She corrected the problem quickly, but not quick enough.
2) Sometimes you may not be sitting on any fabric when your skirt is very short. Case in point, last year my friend at Trophy Life and I were at a college football game. The girl in front of us wore a skirt so short the pockets hung out of the bottom. When she sat down the bottom of her skirt was parallel with the top of the bleacher. It must have been cold.
3) Be prepared for short skirts to blow in the wind. I myself have never witness this, but I just heard two separate stories from eye witnesses this weekend that were flashed by short skirt wearers walking/bending over in blustery conditions.
Having said this...I think there are women who wear short skirts well. I just think a woman needs to be advised of the hazards before putting on her favorite mini.