10 Doctor Who Easter Eggs Hidden In Plain Sight
2. The London Underground
The Christmas Special The Snowmen sees Matt Smith's Eleventh Doctor come up against Dr Simmion and The Great Intelligence, the latter of whom was a villain who had faced off against the Second Doctor. However, in a clever easter egg here, it seems as though the Doctor himself gave the baddie the very information he needed to stage an assault on Earth.
The Web of Fear was the Second Doctor serial that aired in February and March of 1968. In this story, the Great Intelligence, along with his robotic Yeti, returns to bother the Doctor, Jamie and Victoria by staging an invasion of London via the Underground. These events take place after the meeting between the villain and the Eleventh Doctor, during which the Doctor told him that he considered the Underground to be a massive weakspot.
Simmion and the Great Intelligence took this reveal and ran with it, although for all of the bad that it did, it brought about a fateful meeting. Colonel, later Brigadier, Lethbridge-Stewart made his first appearance in this serial, becoming one of the long-serving friends of the Time Lord. The Doctor may have caused himself some bother but he also inadvertently gained himself a friend for life.