The Mann vs. Machine update, released in August of 2012, had been teased by Valve for almost 3 years:
• In October of 2009, during the Haunted Hallowe'en Special, the will of Zepheniah Mann, the father of Redmond and Blutarch Mann, the owners of Blu and Red, respectively, showed that he had an unknown third son, who was revealed during the Mann vs Machine update as Gray Mann, the one sending robots to destroy Mann Co.
• During the 119th update several months later, a teaser video was released for the Engineer Update. In said video, multiple robot heads of the Heavy and Demoman classes could be seen in the background. These would later be seen as the heads for their respective robots sent by Gray Mann.
• Much later, during the Christmas 2011 update, a hidden back room, accessible only via cheats, was found in the newly added map "Foundry". In the room, it shows a cowering Heavy and Scout looking at developed pictures showing several of Gray Mann's robots. Besides these pictures, there are also 3 numbers in the map, "13 22 13", which in alpha numeric code translates to "MVM", the initals of the future update.
• During the Pyromania update 6 months later, the also newly added map "Doomsday" included another room only accessible with cheats. In this room, a disappointed Spy can be seen with the same 4 pictures and a map showing a town, a factory, and a farm. These 3 places would be the 3 MVM maps, Coaltown, Mannworks, and Decoy, respectively.
• 2 months later, the actual Mann vs. Machine update was released