I remember an episode of the X-Files did it.

Pretty good episode too. It was the one about the ship and there were no cuts.

The very beginning of Star Trek III: The Search For Spock used several aspect ratios, if you count each frame of the slow zoom from the Spock death scene from Trek II (starts out small, then zooms into full view).

The film started out in 1.33:1 when the clip from the television show is played then going to 1.85:1 and ending up in 2.35:1 for once Tim Allen travels to space.

The DVD is presented in only 2.35:1.

Split screens: When Harry Met Sally, Snatch, Run Lola Run

Split screen...

Superfly

which seriously needs to be on dvd already, you dig?

Multiple angle...

Rashoman ;)

Narc uses some split screen effects.

Timecode is shot to video with the screen split into quarters showing 4 different cameras for practically the whole move.

Woodstock uses different AR's and split screens.

Mad Max 2 (The Road Warrior) opens in a small 1.33 type frame and then widens out.

My all time favorite for best use of split and multiple screens is the original Thomas Crown Affair (Steve McQueen/Faye Dunaway)

Timecode, is that way the whole movie.