All the time. Another related subject is when someone bumps an old thread, you see that you've contributed to it, but have absolutely no memory of having done so - or even having a clue about what you were on about.
I have read through old posts and as I am reading, I will occasionally see a comment that I wholeheartedly agree with 100%, so much so that I could have written it. Then I will notice that I was the one that wrote it. LOL