I totally get this, OP, though since I'm not a big fan of most AUs (high school/college ones in particular) my pet peeve is when someone does a flashback, prequel or origin fic where the character is definitely younger, in an earlier time period. I've seen people write fic about a character at an age when it would've been like... 1989, and they're carrying around small cell phones. I don't know if it's down to the author being too young to remember that era, or too lazy/careless to research, or both.