Math Tutors & Teachers - Most Common Errors?

I'm a math tutor (and occasionally a substitute teacher). As I'm sure many of you also grumble about from time to time - I keep seeing high school students making elementary errors (elementary school errors) in their mathematics. In particular, I find I need to spend at least 1/2 my time re-viewing concepts that should have been learned years earlier.

Negative numbers - an almost certainty of a mistake to come.

Manipulating fractions - 'simple' errors - that give bizarre results.

Exponents - for some these are a real bug-a-boo.

I'm trying to put together a 'Top Twenty' or 'Top 50' error-sheet - to give to the parents of my students - and to guide me toward making a 'pre-tutoring' test sheet - to see what simple mistakes they tend to make.

What are the most common, or most irritating mistakes that you see from high school students in pre-algebra, algebra, geometry, and calculus?