Many Gene Deitch-directed Tom and Jerry shorts due to the deranged, limited animation.
There's also the "Heavenly Puss" short where Tom gets sent to Hell, even after getting Jerry's forgiveness (though it turns out that it was All Just a Dream.) That probably scared a few kids rather than amusing them. Matt Groening (the same man behind The Simpsons, Futurama, and Life in Hell) even cited this as one of the darkest things he's seen (and this is the man behind such brutal Simpsons episodes as "Boys of Bummer," "Miracle on Evergreen Terrace," "Bart Sells His Soul," and "Homer's Enemy").
Also in "Heavenly Puss" is the moment where the gatekeeper calls the name of Fluff, Muff and Puff, and we see a sack hopping along the floor accompanied by wet sounds before it pops open and three mewling kittens emerge. If the image of three kittens whose owner tied them in a sack and drowned them in a river wasn't enough, the gatekeeper's resigned response to it implies he has seen the same thing a lot.
"Southbound Duckling" ends with Jerry and Quacker in a beach thinking they've defeated Tom. End of episode, right? Not so fast; Tom is right behind them and traps them under a bucket. Then he pulls down an umbrella to hide both him and the bucket, laughing evilly. Common enough sight in the middle of these cartoons, but in this case, that's where the episode ends...