Back in high school, I was at this party and I saw a girl from one of my classes who was always flirting with me but we ran in different circles so we never hung out before. We started talking and drinking and the whole time she kept trying to include her friend in the conversation. Not reading the situation properly, I thought she had her friend there for safety or whatever or maybe she wasn't really feeling me but was trying to play matchmaker with her friend. Fast forward a couple of hours later and I'm dancing and kickin' it with this chick and she gives me a kiss and says "let's get out of here... can you walk me home?" (remember this was high school and I didn't have a car yet). I was thinking "I'm in!" until I see her girlfriend come walking up saying "oh, are we going to leave now?". The whole walk to her place I'm thinking how am I going to get rid of her "cockblocking" friend and she hits me with the "my friend is crashing at my house tonight". Vexxed, I drop her off at her place and make up some excuses about how I got to get going, and walk on to my house (which was about 15 minutes away).
Come Monday back at school, I bump into the same girl on the way to class and she says that I should of stayed at her house. Apparently, her parents were gone away that night and she had worked out a plan with her girlfriend to get me in bed with the two of them. She was testing the waters to see if I was into her friend too because she was inviting me into a threesome and I was too stupid to read the situation.
My friends from back then still clown me about it to this day... so young and naive I was.