Let's talk grammar.

Grammar is something I notice.
I'm not the grammar police or anything, but I notice if a person I talk with a lot has a favorite phrase or a certain rule that they consistently either follow or don't follow. Blogging is interesting because when the words are all spelled out you see quirks in the speech patterns of your friends that you wouldn't be likely to notice aurally.
Now, after I've just made the claim that I am not an officer of the grammar police, I'm going to list the grammatical errors that get under my skin like an itch I can't scratch.
1- "Fred and I's" Generally, I think you would just say "our" but in the case that your audience didn't know the identity of the other half of that "our" you should say "Fred's and mine" rather than tack an "apostrophe s" on the end of I.
2- "My sister-in-laws surprised me" It should be "sisters-in-law" Putting the "s" on the end of "law" makes it sound like "sister in law's" as in "The party will be at my sister-in-law's house"
3- Ok now, this is the big one. I hate, HATE, the interchanging of the words "then" and "than".
It is very common to hear people say "other then that" instead of "other than that". It prickles a bit when I hear that one, but it's such a common mistake I'm rather desensitized to it. It's when the switch happens the other way that it really gets me. Example "and THAN I went to bed" When I come across this, my muscles tighten, my lip curls and I start sucking in air like it's the last chance I'll have to breathe. Inside my head I'm screaming "THEN, THE WORD IS 'THEN', 'AND THEN I WENT TO BED' FOR HELL'S SAKE WHY? WHY WOULD YOU SAY 'THAN'?"
I'm not sure why it is I hate that so much but I really really do. If you say/type that, I apologize if my honesty about my reaction has offended you. Please know that once I get over the initial reaction, I remember what a delightful person you are and put that pet peeve of mine behind me (until I happen upon it again anyway)
Now, here's the fun part. Tell me what you grammar peeves are. I really love to hear what bugs other people. For example, my college roommate Annie's mother can't stand to hear people say "these ones" or "those ones" it should be "these" or "those" the word "ones" should not enter into the sentence. That's a mistake I never knew I was making until Annie told me how it bothers her mother. I've tried to correct it in my speech since then. I can't guarantee I've been wholly successful in the endeavor but I have tried.
So tell me, tell me, tell me what bugs you. I'm asking out of interest, so if your peeve is something I do, go ahead and say it anyway. Don't be afraid to call out my favorite mistakes.
By the way, this is a contest. There will be a prize for my favorite response. Extra points if I made your most loathed mistake in this post.