My wife's worst nightmare.
On the other hand, I love that look of triumph when my child is climbing down, or up, stairs for the first few times. That grin and look of growing confidence gets me every time, and climbing stairs is one of the first times you see it.
2. That moment when your son is falling asleep, and he starts hitting himself in the face to stay awake. -- "Why are you hitting yourself? Why are you hitting yourself? Oh...right."