When I was around 10 or so my dad decided to sell his van. A lady from Craigslist was interested, and I wanted to come along because I liked the van and wanted to be there for it's "last moments" with us. Or something.
So my dad let me come along. He's explaining all the features of the van, talking about mileage, service history, etc, things seem to be going well, when my stupid little ass pipes up for some inexplicable reason: "Hey dad, remember that one time the van stalled on the highway and left us on the side of the road?" (or words to that effect). Not satisfied with my contribution to the conversation, I went on to talk about another time the van had crapped out in some way. I remember knowing at the time that I shouldn't be doing this, but I didn't know why I was saying it or why I couldn't stop.
Needless to say she did not buy the van. My dad took me aside later and calmly explained that you shouldn't talk about how awful a thing you're trying to sell is.
It was a good van too. We weren't selling a lemon, the issues had all been taken care of. Maybe I just didn't want my dad to get rid of the van, or maybe I felt left out of the conversation and wanted to be included.
Kids are stupid.