10 Hidden Video Game Sequel Clues You Totally Missed

8. A New Portal Achievement Foreshadowed Portal 2's Reveal

Valve's legendary puzzle-platformer Portal was first released back in October 2007, and almost two-and-a-half years later on March 1, 2010, Valve added an unexpected patch to the game.

The patch notes puzzlingly read, "Changed radio transmission frequency to comply with federal and state spectrum management regulations," allowing observant players to discover that 26 clock radios had been hidden throughout the game's test chambers.

Taking these radios to the correct location would allow them to broadcast a hidden message, and doing so for all 26 radios would unlock the "Transmission Received" achievement.

But more to the point, each of the radios transmitted their secret message in Morse code, all of which were part of an alternate reality game, and appeared to suggest that malevolent A.I. GLaDOS was rebooting.

An additional patch added on March 3rd then showed protagonist Chell bring dragged back into the Aperture facility. This was widely interpreted to be a hint at the existence of Portal 2, and on March 5th, the game was officially confirmed.