I do this! Fortunately, my betas usually catch it.

There are two reasons:
1. I'm writing more than one story at once, and bouncing from one to another gets me mixed up.
2. I'm writing in more than once place, usually getting brilliant(ish) ideas on the bus or in the store or at work and scribbling them down,a dn then adding them into the main file. And then realizing, damn.