Not nearly as bad as most of the stories here, but my sister has been driving me up a wall every time we try to plan something together. These days it's mostly visits to our mom (we both live in PA, mom lives in FL.) Since we only go down once a year or so, we usually make a pretty big deal of it, staying in a hotel, going to an amusement part, etc. My one big expensive trip for the year, but I still try to keep in under control. This last year we were going to Tampa, spending two days there before driving back to Mom's place for a few more days. My sister and I agreed to split the hotel costs 50/50, and Mom bought the part tickets. Flights we booked separately, since my sister and I usually fly out of different airports. We got a pretty decent deal on the hotel room, and I thought we were all set.
It's not until I get down there and get to the hotel that I discover that she's changed the booking from one room to two, so her husband could come down and join us a day later. Didn't think she needed to let me know that the bill was going to be double what we agreed. >:(
That was the biggest one recently, but it seems like every time we do something together something like this happens. She's always saying, "Why not, you can afford it, you make more than us anyway." Guess what, that means that you CAN'T afford it, not that I CAN. Of course, I technically can, but the reason I can is cause I don't.