So, devoted xkcd fan that I am, I nevertheless missed this one: its April Fool's joke this year.
The comic was called "Umwelt". And the title tooltip (nearly all xkcds have them, and they're often the funniest part of the strip) defined it:
Umwelt is the idea that because their senses pick up on different things, different animals in the same ecosystem actually live in very different worlds. Everything about you shapes the world you inhabit--from your ideology to your glasses prescription to your web browser.I don't really recall what cartoon I saw on April 1: but I know I didn't explore it enough to think of, say, opening it in a different web browser.
It turns out that you saw a different comic in different circumstances. Different web browsers or devices (computers v. mobile phones, for instance) gave different comics, but others were region specific, or tied to various other things.
I have not -- oddly enough, given not only what xkcd is like, but what its fans are like -- a definitive list of all the variations, or even a definitive count of how many there were. Granted, it's complicated: some variations were minor alterations of others, some were entirely different cartoons.
But there are a number of discussion threads and other link pages that link to a lot of them. This article gives a good introductory sense of what sort of variations we're talking about. Then the relevant discussion threads on the xkcd forums and explain xkcd both had links to jpgs of a lot of versions. This video shows the way a particular cartoon (it was the one I saw just now, on Firefox on Mac in upstate NY) alters when the browser window is resized. This google docs roundup has a lot of different versions. Perhaps the most complete lists of links are those in this follow-up post on explain xkcd.
But what's most amazing to me about this whole thing is that, while some of the comics are not quite up to par, and some are minor variations on a particular idea, many are actually good enough that you wouldn't be surprised to see them as a regular xkcd installment (including the basic theme under several sets of those variations). Which is to say: this joke must have been a really tremendous amount of work -- for a joke which insured that most people would miss most of the comics.
Which, in my book, is pretty impressive.
I've stuck a few ones I like into this post, but really -- if you like xkcd, you should go explore. They're fun.
And if you don't know xkcd, check it out. (Just hit the random button, ignoring any cartoon numbered less than 100 or so -- he took a while to hit his stride.) Especially if you understand the meaning of the number in the title of this post (hint: look at the number of the comic this post is about.)