Four variant teaser trailers.
They're the same trailer, and differ only in the half-asleep mutterings uttered by Mike/Billy Crystal.
When Mike and Sully are going to work in the beginning of Monsters, Inc., Mike tells Sully that Sully's been jealous of his good looks since the 4th grade. But this film depicts them first meeting in college.
In the beginning of Monsters Inc., Waternoose is depressed because, among other things, he tells Sully that the newest batch of scarer recruits stink, and later, in the middle of the film, he has to have Sully give a demonstration to the incompetent scarers as to how to scare kids. This made it seem as if good scarers were a rare breed, and/or that it is an applied skill that has to be taught to them on the job, with finding scarers with inherent talent being a matter of finding a needle in a haystack. But this film depicts scarers as being so ubiquitous , and the field so highly competitive, that only a select few who decide to major in the field in college have a chance of being admitted to Monsters Inc.'s scare program. Since Roz states at the end of Inc. that the CDA's investigation took two and a half years, and she was presumably not undercover at the company yet at that point, that means the first movie can't take place more than two and a half years after the second one, and probably considerably less than that. So what happened to the field and its workforce between the two?
You know, that could be the subject of a complete third movie.