Ready to pull my hair out

Someone with whom I'm working and I had the following conversation:

OP (other person): Now we have A covered. What about B?
Me: Don't worry about that. I wrote that out for someone else. Just focus on B.
OP: But I can do B, too.
Me: You can if you want to, but you don't have to. I think the more important thing is getting A done.

A couple days later, we were meeting with someone else (whom I'll call IP for important person).
OP: We're planning on doing A and B.
IP: Are you sure? You know that we don't have the widget for B yet. Will that hold you up?
OP: No, getting the widget for A will be the difficult thing.

And what happened this morning, less than two weeks later?

OP starts complaining about how the widget for B isn't ready, which is holding him up, and then has the audacity to claim I TOLD him to do B.

I am amazed at how some smart people can be so dumb.