DH was the first to pick up on this. I think I've annoyed a lot of people over the years as a result of it.
We agree a course of action, sometimes I've even said I'm going to go and do xyz. Then for whatever reasons in my head I decide I'm going to do something different instead, sometimes something that is the opposite of what I've agreed with someone.
I don't know how I've done this for so long. DH knows now that I often do this without even realising but I'm trying to become more self aware. Not least because it'd drive me nuts if someone didn't do what we'd just spent a while talking about.
I think having a poor short term memory doesn't help. Sometimes I have genuinely forgotten what I've agreed, other times I think on impulse I decide something else is better.
Anyone else?