Here's one that broke the deal on a friendship recently: Making oneself look generous at another's expense. This last incident broke this camel's back:
Friend invited me to go to "The City" with her. I'm waiting and ready at the time specified, friend arrives late with strange woman in tow - hadn't been mentioned. Friend drags me to all the stores Stranger wants to see, then insists on expensive restaurant. After lunch is finished, Friend insists that Stranger must accompany her to the restroom and disappear. Bill arrives, I wait for several minutes with friend and stranger returning. Finally, I pay the whole $100+ check, then leave the restaurant, to find Friend and Stranger had completely left the restaurant and had been to a store or two - didn't tell me. Friend had decided that since she drove (gas + $5 toll), I was to buy lunch - without informing me beforehand. The whole trip was to entertain Stranger, and Friend was beaming with the milk of human kindness at her own generosity. Friend tried to stick me with the parking fee as well, but Stranger pulled out her wallet and paid for that ($28).
Suffice to say, that's the last time I ever accept an invitation from Friend again.