The scene in episode 2 where Haruka, Mamimi and Naota are all talking and the animation style has suddently changes to a more abstract style.
The grayscale style used for collisions, which comes up a few times.
This was incredibly hard to animate, and the second one is brought to an abrupt end by Kamon, who breaks the fourth wall to point out "Why can't we be a normal anime!? The animators asked us not to do another manga scene."
The "manga sequences" in episodes 1 and 6, during which the standard animation style is replaced by pans across (semi-animated, with voice-overs for the dialogue) manga pages.
In episode 5, when it flashes back to Amarao asking for a "manly" haircut in a hair salon, it's done in the same animation style as South Park.
BLACK AND WHITE PENCIL ANIMATION SERIES
Done extremely frequently in FLCL, more and more as the series goes on.
Repeatedly occurs in Magical Shopping Arcade Abenobashi, with each world often having its own unique art style.
in a scene parodying the survival action-movie fad from The '90s, a Puchuu cuts Excel's rope and spits out, "Burn in hell."). Sometimes it happens without them being killed, if the gag demands the nasty phrase (e.g. The also utter something mean-spirited/action-movie-ish when it happens (reflecting their true, evil nature).
And then there are the Puchuus, who suddenly change from Ridiculously Cute Critter to something Golgo 13-esque when you kill them.
This happens many times in Excel Saga, most notably in episode 17, where there are so many rapid-fire shifts (from Looney Tunes to Disney to DC Comics to The Simpsons and beyond) that it's impossible to keep track of them all.
Many scenes in anime (especially Gag Series) involving fighting will often make a visual reference to either Fist of the North Star or JoJo's Bizarre Adventure.
Almost the entire last episode of Gunbuster was deliberately animated in black and white-on color film stock.
Each art style is different from the last, and many dip into the realm of experimental animation.
In Hyouka the art style changes during the sequences where the characters break down the mystery.