It is a glitch in the matrix. Agents will be visiting you shortly to erase this troublesome memory.
Seriously though, I know exactly what you mean. Some times this happens to me with conversations as well. Like I might *know* for a fact that someone mentioned a certain detail, or asked a certain question before, etc., but then nobody knows what I am talking about when I bring it up, like it never happened...
And then I wonder if dreamt about it....
Humans in general are not very reliable though, but I gotta make do with what I've got, don't I?