Perspective

Used to fantasize about these being used to play local multiplayer without having to split the screen
- Iirc Sony did this with the PS3 and their own PlayStation branded TV.
  
  Sony patented this in 2010 to allow people to simultaneously watch two different videos on the same TV.

So I have a lazy eye and went to a 3D Aquatic documentary on a family trip. Basically watching it with one eye because of partial functionality in my bad eye.
Something happened like 30 min in during a scene with jellyfish and my brain turned on 3D and everything popped out me. Totally tripped me out.
Doc always said my brain relies more on my good eye and I think it decided it needed the bad eye during the flick. Pretty cool.
- Makes me think of the asdfmovie when someone accidentally turned on the color.
  https://youtu.be/qW0pYFDFlTE
  
  Accurate. I gripped my armrests and pressed my back into my seat in silent shock.

Tilting your head shouldn't make a difference as 'modern' (as in the ones that cinemas started using fifteen years ago) 3D glasses use clockwise and anticlockwise circular polarisation filters, and obviously, turning something 90° doesn't change whether it's clockwise or anticlockwise. Other kinds of polarisation filters do care about being rotated, which is probably where the artist got the impression it applied to 3D glasses, but it would be dumb to try and use that kind as obviously, people tilt their heads.
- Huh I guess I haven't been to a 3d movie in quite some time. I definitely remember had tilting used to work
  
  3D IMAX was way later to using circular polarizers (if they ever switched). Were you only watching 3D in IMAX?
- Wouldn't it still look weird because the two images are offset in a different axis than your eyes?
  
  It can make it look a bit weird (and be one of the things that makes people nauseous), but most people's brains are good enough at figuring it out that it's not a major problem. It wouldn't make a difference to the situation in the comic, though, as in that frame, they're talking about making both lenses match to pick between two 2D movies, so there'd be no offset anyway.
- I don't know what modern means here, but the cinemas I go to still use the same glasses as from the beginning of 3D movies.
  
  Anaglyph 3D (with the red/cyan, or lower-quality red/blue filters) has been around since the 1800s (3D films predate talkies by decades), but was much more of a gimmick and wasn't used for big-budget serious films as it ruined the colour quality, mainly being used for 1950s B movies. There were a bunch of other methods used between the 50s and 2010-ish like regular Polaroid filters (which did stop working when you tilted your head like in the comic and were a hassle for other reasons) and active shutter (which relied on expensive and heavy glasses with electronics in). The newer kind that relies on circular polarisation became available since the turn of the millennium, with Avatar in 2009 being the film that made most cinemas buy new projectors. Those glasses made 3D films viable as the standard for a few years, before people generally decided that most of the time, being 3D didn't add enough to the viewing experience to be worth paying extra for and studios decided it wasn't elevating their art enough to justify the extra production costs if people weren't going to pay a premium.

I forgot 3D movies were still a thing. I only ever saw Avatar and Coraline in 3D.
- Even after the camera for it was invented (for Avatar, iirc) most studios just filmed in 2D and converted it to 3D, which looked awful. So on the one hand you have a small number of movies, like Avatar and Coraline, filmed in 3D and looking gorgeous, but on the other hand you have a ton of movies filmed in 2D and hastily converted to 3D to cash in on the success of the ones filmed that way. Those movies looked awful and pretty much killed the trend.
  However, the theaters still had the equipment to show them because it's not like there's really anyone to sell it to, and the 3D cameras are still out there, too. So every once in a while someone decides to put that equipment to use. It's not many, though, and most 3D movies are still 2D conversions, so you don't get much out of viewing them in 3D, and the conversion process and the display tech can actually make them look worse than just watching the original 2D version.

Here's fun trick: rotate the glasses 45°, see which movie you get to see, it could go either way!

The first time I saw this concept was in the sci fi novel Who Goes Here? by Bob Shaw back in 1977. The main character goes to a theatre that shows x rated adult films and children’s movies on the same screen, and what you see depends on which glasses you are wearing. Some teen tricks the main character into trading glasses with him if I remember correctly.

It's a convoluted way of reinventing an eye patch.
You don't need to obtain and install a polarized filter when you are only using that filter to block all light going to one eye. Tape a piece of black paper over one lens, ya dummy.
- You the dummy. The modified glasses would make both eyes receive the same polarized light thus the same picture, not block the light.
  
  You're right!
- Well, good thing this comic isn't talking about blocking out all light to one eye. Why would you want to watch a movie with one eye blocked anyway? Sounds horrible.