Inuyasha: Every single sit. Though my personal favourites to date are(spoiler warning):
Where Kagome's bathing, and Inuyasha is caught spying on her from atop a cliff. "Sit", Inuyasha falls of cliff face first.
Whenever a wooden bridge is involved. "Sit" till wooden bridge gives way, Inuyasha falls face first into river below.
Finally, when he plans on destroying the Bone Eater Well with a huge boulder, so Kagome yells "Sit" so many times it silly, then the fact he is no longing holding aforementiond boulder leads it to fall on him, putting his back out.
Naruto: Lesse, mostly the first two episodes, because of the reactions to 'Sexy Jutsu' and 'Harem Jutsu'. Those two jutsu, by Flygon. Haven't laughed that hard in a while.