One very popular theory is that Jeremy Fitzgerald is the victim of the much-debated Bite of '87. On Night 6, Phone Guy says, "Uh, we have one more event scheduled for tomorrow, a birthday. You'll be on day shift, wear your uniform, stay close to the animatronics, make sure they don't hurt anyone, okay?" This leads people to believe that Jeremy did as he was told and, having stayed close to the animatronics, wound up being attacked and bitten by one of them.
Supporting this theory is the fact that the animatronic characters had been acting strangely recently, having been possibly tampered with. "But the characters have been acting very unusual, almost aggressive towards the staff. They interact with the kids just fine, but when they encounter an adult, they just...stare," Phone Guy says, which may imply that Jeremy, being so close to the animatronics, could have easily made him a target.I have also came up with another part for this theory to, because Night 7 is a bonus night and seeing the phone guy was now dead there is no speculation that it was Jeremy in that office.
