I seem to have more the older I get, but here are two that currently are driving me crazy:
What has happened to those 2 little letters, "er" to describe something that is biggER, longER, prettiER, etc? Everyone seems to now say "More long", or "More pretty", or whatever? Huh?
And not knowing the difference between "Fewer" and "Less". Less is when, say, a person has eaten half of their sirloin. There's "less" of it than there was when they started eating. "Fewer" describes when a group of items, say, like marbles, has been decreased in numbers. Like, "I've lost most of my marbles so I now have 'fewer' than I did when I was in my 20's".
Anyway, I guess it's syntax errors that are my pet peeves! That and spelling!