Revealing mistake: When Nelson first undergoes the experiment, the doctors use a small mask to put him to sleep. When he is supposedly waking up, Rachel asks Randy to help her with oxygen. He hands her the same small mask they used the first time ie. anesthetic. Just for a second she has a look on her face which says "this is the wrong mask" and she tries to hide it with her hand. For the whole rest of the film, the other doctors use a bigger mask when giving oxygen.
Revealing mistake: Rachel's father's suicide: After he heads down the stairs (during her death experiment), we suddenly hear a popping sound like a gunshot, as the girl follows her mother outside, to see glass littering the pavement. Then the camera flipping to shots of an old pickup parked on the curbside. And a single bloodied gunshot hole through the windshield. Which would not have happened within the 5-second time, between the stairs and the curb.
Chosen answer: No hallucination. It was Halloween night, so everyone was acting a little crazy. That's why one of the med students shows up in a skeleton suit. Going further, the 'hum' of the bikes were used not only to throw the viewer off as the same humming noise Nelson describes with coming back from the dead, but to confuse Nelson with death and reality and these new sensations/feelings he is experiencing since being brought back to life. The cyclists are used, in a sense, to scare Nelson and viewers. The same 'hum' can be heard when Nelson first finds Billy Mahoney in the dingy tunnel.