The Bite of ‘87

The Bite of '87 was an incident that occurred in 1987 at Freddy Fazbear's Pizza it was briefly mentioned by the Phone Guy in the first game.

Overview
The details of this attack (and the identity of the animatronic responsible) were not given, but it allegedly caused the loss of the frontal lobe of the victim's brain. The victim survived the attack.

The incident contributed to a loss in the restaurant's reputation, which resulted in a major drop in business.

As a result of the incident, the animatronics were no longer allowed to wander around the building during the day; the management decided to limit their free-roaming mode to nighttime, so as to "prevent their servos from locking up", as stated by Phone Guy.

The Bite of ‘83
A similar incident, referred by fans as "The Bite of '83", was shown in Five Nights at Freddy's 4 during a cutscene after completing the Night 5. The whole events in the game took place in 1983. The main protagonist of the game, a child who is being thrown a party at Fredbear's Family Diner, is being bullied by several older kids and his older brother who want to scare him. To scare him, the gang gets the victim closer to Fredbear's stage and makes Fredbear "give the child a kiss" by shoving him into Fredbear's mouth. Whether by accident or not, the protagonist's head gets stuck in Fredbear's gnawing mouth. The machine's mouth then crunches shut on the victim's head. Unlike the Bite of '87, the victim of this incident didn't survive.

Freddy Fazbear’s Pizza Place Bite
Another similar incident took place off-screen in Freddy Fazbear's Pizzeria Simulator, according to one of the lawsuits. One lawsuit reports that the male child's head is bitten by the unidentified animatronic and suffered severe injuries, but his frontal lobe remained connected to his brain. Despite this, the doctors are unsure how long the frontal lobe will remain that way.

Theories and Speculation

 * Some fans have speculated that Foxy may have been the animatronic responsible for the bite. This is considered most likely by fans because he appears to be the most capable of doing so with his seemingly sharp and broken teeth.
 * In Foxy's dwelling place, Pirate Cove, there is a sign that reads "Sorry! Out of Order" and Foxy himself is in a state of disrepair, possibly suggesting that the attraction was shut down after the incident. Foxy's jaw also seems to hang loosely, as if it were damaged through excessive force. However, when entering the Office to attack the player in the first game, his jaw is always closed, then falls open. However, the jaw could have been closed due to Foxy's running (his jaw can be seen flapping).
 * The second game, however, seemingly rules out Foxy (as Withered Foxy specifically) as the attacker, as he and the other withered animatronics were most likely in Parts/Service when the bite took place. This can also be disproved by the fact that Foxy is still present in the first game - if he had caused the bite, then it would seem very odd for the owners of the company to keep him.
 * Still other fans speculate that Freddy Fazbear himself was responsible for the bite. On the posted rules in the East Hall in the original game, one of the rules listed states, "Don't touch Freddy." This rule may have been enacted after the bite; however it could also be because the killer disguised himself as Freddy to kill the children, the fact that Freddy is the main animatronic, so he represents this rule for all the animatronics, or due to an incident not mentioned in the games.
 * There are also two instances of faint handprints or hand-shaped impressions on Freddy's face, implying that someone may have been struggling against him in some way. However, these could also be Phone Guy's handprints, because as heard in the phone call from Night 4, Freddy's song plays when Phone Guy dies, leading some to believe that Phone Guy could have tried to defend himself from the animatronics. Also, when the bite occurred, Freddy most likely had another head, as in Five Nights at Freddy's 2, he possesses a different mask than the one which has handprints on it.
 * Another popular theory among fans is that Mangle was the cause of the bite, as its jumpscare involves it swinging from the ceiling of the Office with its jaws open, going straight for the player's head. Upon inspection, it seems that Mangle attacks the upper area of Jeremy's head, where the frontal lobe is located. This further strengthens the possibility that The Mangle administered the Bite of '87 in a similar fashion to its death screen. This is reinforced by the mention that the new animatronics were being scrapped, something that would be likely to occur if one of them had caused the bite.
 * Another big piece of evidence that Mangle was behind the bite is that it would have a reason to bite a child, as it is known that the animatronic is dismantled daily by the kids and that the staff eventually stopped repairing it, leaving it as a "take apart and put back together" attraction, which could have led it to either hate children (if such a thing is possible), see a child as a possible threat, or otherwise become unstable. This is questionable though since on the fourth night, Phone Guy states that "[the animatronics] interact with the kids just fine, but when they encounter an adult, they just...stare." However, since the actual victim of the bite is never specified, some believe that it wasn't a child at all, and if an adult had received the bite, then it would make sense for the animatronics to respond that way towards adults.
 * This can also be backed up by the fact that at the end of Night 6, all the toy animatronics (including Mangle) were scrapped due to apparent malfunctions. If Mangle was the culprit of the bite, it would make sense for the owners to scrap the toy animatronics, since the company may have feared that the other toy animatronics could experience similar malfunctions. If Withered Freddy or Withered Foxy (the other two most-likely culprits) had caused the bite, it wouldn't really make any sense for the company to have scrapped the toy animatronics.
 * It is also worth noting that Mangle was the only active suspect at the time of the bite, as Withered Freddy and Withered Foxy were presumably in Parts/Service, this could leave Mangle as the only animatronic capable of producing the bite. Despite this, many fans still believe that the bite was somehow caused by a different animatronic, leaving the true culprit unknown and up to debate.
 * While the bite is still debated about which animatronic did it as the answer is still unconfirmed, the most likely culprits are: Withered Freddy, Withered Foxy, and Mangle.
 * Withered Freddy is believed by fans that he was the one responsible for the bite, considering how big his upper and lower jaw are and how his jaw is big enough for a whole human head to fit into it. However, this seems to be very unlikely as Withered Freddy was presumed to be in Parts/Service when the bite had happened. Also, if he was the one who committed the bite, this would have caused the victim to be killed, rather than surviving.
 * Withered Foxy, just like Mangle, is heavily believed to be one of the culprits as well. This is evidenced by how long his jaw is and his particular behavior in Five Nights at Freddy's 2. In Five Nights at Freddy's 2, when he's present in the hallway, he can be seen with his hook up as if he was going to dismember the guard's head off, also during his jumpscare, he gaps his mouth as wide as he can, as if he was trying to bite the guards head off. However, just like Withered Freddy's case, this theory seems to be debunked, considering the fact that he was presumably in Parts/Service when the bite occurred.
 * Concluding this evidence up, Mangle seems to be the main suspect for the bite, however it is still clearly unknown and still unconfirmed of which animatronic in the pizzeria did it.