At my first contracting gig using a body shop way larger than I was, well, they'd kinda sort of start thinking about paying me 30 days after they got my net 30 invoice. This /really/ bothered me. In fact I made a lot of unprofessional noise to the client, and ended up staying home, once, until they paid me. The thing was, I was really worried that they weren't going to pay me at all.
It did not occur to me that this was just standard operating practice, and that they were planning on paying me; they just wanted to stretch it out a bit.