The other day, while walking down the street, I heard somebody say to their companion, “Sometimes I don’t want to spend an evening out with my colleagues. Sometimes, I’d rather be in my pyjamas.”
I was close enough to have said to him, “Why can’t you do both?”.
But I didn’t.
gamboling.co.uk 
I know what you mean about something coming to your lips and then it gives you the slip. People have said things to me and then that night, I lay in bed twisting my sheets into sweaty knots going, “Why didn’t I say that when I had the chance?”
It’s an annoying habit that one. I do tend to think of great things to say… on the way home. Sometimes, in my darker moments however, I wonder if the only reason they seem good comments is that they forever go untested by the public!