As a college composition and literature instructor, I can safely say that most of the pet peeves already listed on this thread are ones I go over in class every semester because I see so many of my students making the same errors. Conscience and conscious, affect and effect are two I believe have already been mentioned. Many of these are homonyms, but as I teacher of writing, I have to remind my students that "sounding" right is not enough.
Some other "swaps" that all-too-frequently appear in college essays:
accept and except
allusion and illusion
board and bored
elicit and illicit
loose and lose
passed and past
patients and patience
then and than
through, threw, and thorough
weather and whether
whose and who's
you, your, you're
(I've alphabetized the above list -- some occur more often than others, but all are common.)
The other side of the coin, however, is not pleasant. When people outside of my classes know I am an English teacher, they tend to try to write as properly as possible, but some probably avoid writing me anything, although I restrain myself from over correction. I think they experience flashbacks from some carping English teacher in elementary school and just freeze.
And I am trapped between just writing (typing), not worrying excessively over esoteric grammar rules, and getting stung by people who are trying to catch me in a mistake. For instance, I opened the common problem list in this post with an open-ended sentence, followed by a colon, and ending with each line in the list. I actually stopped and asked myself, should I properly close each line with the period or just leave the flipping thing alone?
There should be a support group for people like me!
I do tell my students the only time I should ever see "It's" used when it does not refer to the contraction "it is" is when they are referring to the Hermann Munster's cousin's possession.
And while possessive mistakes are annoying, sentence faults and comma usage are far more deplorable in my students' essays.
Truly, ignorance is bliss.
