How well do you know them? Is it from work or your personal life. If work, do they sit near you? Within three cubicles? Do they sit in the same meetings you do? Do you talk? Or do you just nod when you move past each other in the hallways?
Are they a friend? Friend of a friend? Someone you met twice/ Former lover? If you know them by association, will they bring up the snub, and will that person bring it up to you?
Have they seen you? Are they close? Where are they going?
Do you have to?