This happens all the time -while jogging,strolling in your neighbourhood,walking in your office(esp if its big),coffee shop, restaurant , shopping malls etc etc - you see a person , he / she looks at you and you start thinking -
hmmm looks familiar
Do i know him
Where we have met before
Know but never introduced
and by the time you decide "to pass a smile" you loose the very precious eye contact with the person .
Well it important to recollect but smile can be passed immediately - its doesn't cost anything even if you don't know the person at all .
When in doubt Smile , n do it fast:)