The moment their arms spun freely in our air, they were doomed — for Man has earned his right to hold this planet against all comers, by virtue of occasionally producing someone totally batshit insane.
Umwelt. The image changes depending on what browser you are using, what OS you are running, how wide your screen is, the height of your screen, where you are coming from, and your geographical location. Known variants include (but are not limited to): Wikipedia, Reddit, the UK, Alaska, Florida, Illinois, Germany, the Google corporate network, MIT, a mobile browser, the Chrome browser, Netscape Navigator, Israel, the Armed Forces, and many, many more.
The 2011 MIT mystery hunt included a puzzle (click "call in answer" for the solution) based on xkcd. So far, so normal. The solution was highly constrained by the round's meta. So Randall wrote a strip based on the answer the puzzle needed to have, and published it during hunt — shortly before the first team unlocked that puzzle.
The evolution of the story is the Holy Shit Quotient of Awesome in itself: what started with very little words as a simple construction of a sandcastle on a beach, proceeded to go increasingly elaborate, to the point of including a trebuchet. It took a month worth of updates for the couple to finish the castle and embark on a quest to find out how rivers and sea work, turning it into a Road Trip story. They encountered a lot of strange stuff along the way and over two months of updates later finally met other people. They were treated to an Exposition Info Dump and a Reveal: all this time they've been on the bottom of what we know as the Mediterranean Sea after it dried up, and the story itself appears to be set After the End in the far futurenote Supported by presence of small isolated groups of people with very varied level of the world knowledge and also via dating by the star map.. And then the Wham Episode followed: The Sea is about to refill again, bringing a Class Zero Apocalypse and resulting in The End of the World as We Know It for our main characters, who are now in a mad rush to save the rest of their people.
The supplemental phenomena of "Time" also deserve a mention. The official forum thread went over 40000 posts long ago, the wiki on "Time" created due to Wiki Rule has over 1200 pages and already the most avid readers have managed to start a Parody Religion... which after several schisms has splintered into several.
Typing 'import antigravity' into Python (Python 3.3, at least), links you directly to this strip.