My top 4 insane Teroies

1.Mike is the Disturbed Survivor of a Childhood Massacre
Occasionally posters change to reveal newspaper articles about a series of children disappearing and having been murdered by a figure wearing a Freddy costume. The bodies were never found, though. Rumor and reports from health inspection, who claim that the four animatronics smell rotten, leads the player to assume the bodies were stuffed into the animatronics. This is their method of killing the player.

However, five children went missing but there are only four animatronics... except for the mysterious Golden Freddy. The strange figure acts more like a hallucination than the rest, and can teleport right into the office under the right circumstances. He still results in a game over if he appears.

Speculation runs high that Golden Freddy is actually a manifestation of Mike's memories of having survived the killer that claimed four other children. Whether his return to the restaurant is him confronting his fear or just the machinations of the haunted robots is anyone's guess.

Another possibility is that Phone Guy is the fifth child calling you for help but still under the power of Freddy and his gang.

2. Mike is the Brain-Damaged Victim of Foxy
The Bite of '87 incident is a something referenced by Phone Guy where one of the animatronics bit the frontal lobe off a child. It's widely assumed this was Foxy, the disused, out of order robot of Pirate's Cove with the sharp teeth. Phone Guy is quote clear that the victim lived.

It's possible Mike is that child. One of the symptoms of severe frontal lobe damage are extreme risk-taking (This job for instance) and the rapid eye-movement seen in your compulsive checking of the cameras. There's also the fact that Mike is paid $4.00 an hour, clearly below minimum wage.

You know who can be paid below minimum wage? The disabled. It's a law designed to incentive employees to hire lower functioning adults to give them something productive to do. In fact, Goodwill was thrust into the national spotlight on the subject when it was revealed they were paying a blind employees $3.99 an hour.

Wouldn't severe brain damage count in this category? Perhaps Mike is the victim come home to face his attackers.

It's all confusing and strange, but utterly remarkable that such a simple game has left enough of a mystery around it to enable such speculation. Hopefully we'll find out more when the sequel comes out.

3.The Phone Guy is Freddy

Aside from you and the four animatronics the only other character in the game is someone referred to as Phone Guy. He calls you each night at the beginning of the level to offer advice on how to stay alive.

Except that he really doesn't. His advice is usually wrong and he claims that the player is really in no danger half the time. He even tells you to leave the room, which would probably be an instant kill. Even on the fourth night when it sounds as if the animatronics are attacking him he exhibits no real panic. The theory goes that Phone Guy is actually the animatronic Freddy Fazbear himself using the same voice box that lets him sing at shows during the day.

On the other hand, there is that weird fifth message...

4.

Golden Freddy's Suit

Another popular theory is that Phone Guy was stuffed into the Golden Freddy suit. This seems plausible, as when Golden Freddy appears in The Office, he takes the position of a slumped over corpse. Adding to it, an endoskeleton cannot be seen inside Golden Freddy, especially since he has no eyes. This also lines up with the fact that Golden Freddy is a Freddy Fazbear suit, making it a very likely candidate to be stuffed with a security guard.