How physically strong they are and how they will hurt you because they have no concept of cause/effect= pain.

hemorrhoids
being tore up from the floor up (kind of goes hand in hand with the above)
The unnecessary messes....They drive me batty. Today for example dd (aug. 09) decided to throw a bunch of shoes out over the baby gate on to the deck....why???? because she could.

This evening she climbed up into a cabinet above the toilet grabbed her older sisters expensive face wash and drizzled it down the stairs to the boys beds.......She is supposed to be in bed.

I know there were more (with 6 there is ALWAYS more

), but those were a couple of examples.
The rest I was pretty prepared for.