"Pink Elephants on Parade" from Dumbo, just... Just look at that scene! It's the scene when Dumbo and Timothy Mouse drink water spiked with discarded champagne and hallucinate all these freaky-looking elephants!
Even though Ralph Bakshi's animated films are more known for their adult material, some of his films, especially Heavy Traffic and Coonskin, have trippy sequences that could be considered mind screw.
Two words that have yet to be said despite being the name of one of the most famous incarnate of Mind Screw to come out during the 1960s: Yellow Submarine. The interior of The Beatles' house is just as insane as it could get. A door opens, and King Kong is about to make off with Fay Wray. George demurely asks "Do you think we're interrupting something? John: "I think so."
Large parts of The Old Lady and the Pigeons, the most surreal moment probably being the ending when the gendarme jumps out of the window and "flies" for just a few seconds.
My God, Felix the Cat: The Movie. Just... just watch it. Reptilian creatures, a magic bag, a half-robot evil overlord, a sentient tear, a dimensional transporter, mice-lizard hybrids, Yellow Submarine-like sea creatures, a Circus of Fear, a forest made of giant hair follicles with head-hunting creatures that are always losing their own heads, evil cubes and cylinders, and a book of ultimate power that defeats the bad guy by being thrown at him. And to top it all off, there's also a German version. This is largely due to a conflict between the two major producers of the movie. One wanted old-school Felix and the other one wanted wild and crazy Ren and Stimpy-type animation.
The Animatrix, a collection of short anime films based off The Matrix trilogy, easily qualify as Mind Screw material.
The Mind's Eye series of videos. In the big picture, the lack of coherence between scenes in most of the sequences is caused by the editors receiving a vast number of submission from hundreds of different animators and trying to splice them together in as thematic a sequence as possible. Then there are the smaller animations themselves, which run the gamut of "weird" by showcasing outright bizarre creatures, physically impossible scenarios, and people whose heads can be literally anything!