Deja Vu, or what?!

Today, as I was taking a quick glimpse of the lecture’s hall, I saw someone I never thought he’d be around at that exact timing. Simply because it’s not his normal attitude to attend such lecture. Anyways, I wonder what made me notice the cloth he was wearing, I mean this is not my usual to notice such things, but so I did.
And actually, still I didn’t pay much of attention to that and turned around to focus with the material discussed. Then the door was opened just after the lecture finished, finding that very same person entering from the door!!! To my surprise he was wearing the very same cloth I thought I saw him wearing minutes ago.
Take into your consideration that the hall got only one door in the front. While I was gazing at the entering and leaving flow of people as I was waiting for my friend. So no way that person left then re-entered.

I know it’s a trivial something to mention but it really made me wonder all day long, how could I see him while he’s not there in the 1st place ?!