Gap Inc. is headquartered on the Embarcadero, with some offices in the building facing Folsom Street. Google also has offices directly across Folsom Street. Employees at the two companies have started posting messages like "Hi" on the windows, spelling out the letters with Post-Its, and the latest prank from the Google side is pretty funny.

The Google kids, being tech savvy, decided to stick a big QR code up in the window, leaving the Gap kids to figure out how they were going to access it. Once they did, they realized they'd been Rickrolled.

"Ugh," writes the poster via Facebook, "How do we even come back from this?!"

The photo naturally made it onto Reddit.

Does this inter-corporate zaniness go on all the time? SFist observed this happening amongst four buildings at Second and Jessie last year. A Redditor tells a similar tale of "a barely-legible YouTube URL" posted in a window four floors up across from the Irish Times downtown a few years ago which also turned out to be a case of Rickrolling.

Another Redditor talked about working at 555 Market and "we would have post-it note battles with the building across from us. Somehow it turned into an Atari theme and we took gold for Donkey Kong."

Previously: Several SF Office Buildings Had A Conversation Today Using Post-its