Say you make specific plans together, then do not show up.
I'm not particularly offended, I'm just mildly confused. This seems like an easy opportunity to say "sorry I'm busy" or some other casual disengagement.
I'd personally consider that extremely rude. I've met people who appear to think it's not a big deal though. They'll arrange something with you but will arrange five other things simultaneously and will choose which one to attend at the very last minute. Presumably they think you're in the same position.
The other day, someone did something I didn't expect. It was something many people have done before; something that I thought of as very normal, but that I in no way understood and had not predicted.
As I said, this had happened many time before, so I wrote it off as "me not understanding people" or "people are weird" for a second, like I usually do, before realizing that "bad at" really means "lacking basic knowledge", which I had never realized before.
And then I thought "I should ask someone who is different from me why people do that, and eventually someone will have an answer."
But many people will have many more questions like this. So, what have you observed people doing time and time again, but never understood? Or something that you only understood after a long time or asking someone about it?
And can Less Wrong tell us, not necessarily why (I for one can make up evolutionary psychology fairy tales all day if I want) but what conscious thought process occurs behind these events?