Here are the best ones, IMHO. Feel free to weigh in with your favorites.
3. They conduct themselves poorly in public.
8. They have absolutely no coordination.
22. They draw on others' faces when they pass out.
25. Things go wrong around them for no apparent reason.
Number 8 and number 25 made me laugh hysterically for probably about 2 minutes straight because they reminded me of an incident that happened years ago when my kids were little. A friend of mine had twins the same age as my daughter and so they often played together. I'm sure these two little boys have since both grown up to be perfectly delightful and entirely normal young men, but at 2 they were both so clumsy and uncoordinated that they scared the living crap out of me whenever they came over.
Our home was a two-story and my daughter's room was at the top of the stairs. This meant that, no matter HOW I tried to anticipate and bring every possible toy or game downstairs for them to play with, there was always something or some reason that they felt necessitated they go up those stairs.
Which meant they had to come down the stairs.
While my daughter at two was like a little monkey and dashed around the house, racing up and down the stairs without a care, these two little Neanderthals would lug themselves up the stairs like it was Mount Everest and they were the only sherpas for an entire expedition, which -- while funny -- was not particularly dangerous. It was the descent that was horrifying. Every time.
As they seemed psychologically incapable of going anywhere without the other, they would always descend the staircase together. Within just the first few top steps, one would invariably hit the hall, bouncing off and into his brother, and the careening would begin like bumper cars in a narrow aisle. Rarely did either of them actually fall. They seemed to use each other to collide into as they rocketed down the steps, yet the possibility of tumbling was never more than one step away. Even at two, these boys were stout and built low to the ground; if I had tried to throw my body in the way to slow the avalanche, they would have just taken me out.
One evening their mom had stayed to chat and the kids had sneaked upstairs. I wasn't aware that they were upstairs until I looked from the dining room and saw Bill at the top of the stairs. Justin, the other twin, mysteriously wasn't with him. The absence of the necessary twin must have caused Bill to suffer from vertigo because he took a few steps, stopped, and began to sway as he stared down the staircase. I arose immediately and indicated to their mother that we needed to grab Bill from the stairs, but before anyone could react, he threw out his hands like a blind man looking for something solid, and then just did a full-length face plant down the stairs. He stuck so hard, he didn't even slide. He just lay there, face down, head and shoulders pointed down the stairs.
Even as the screaming started, it was perfectly obvious that he wasn't really hurt. But his mother never really accepted that as my defense for laughing so hard.
We weren't really close after that.