Well, generally, when I see people I haven't seen in a while, I will say "Hey!" and ask how they've been doing. But, oh man... I do have a story about an incredibly awkward encounter though:

When I was working at the restaurant, I would sometimes get old classmates that would come in to eat, and yeah, some would find their way to my section. Most of the time, it was just the "Hey, how have you been???" and "Good to see you!" type of encounter. However, there was one time when a girl that I got into a fight with in HS came in, and sat in my section. I could still feel that there was a bit of friction between us, but we both behaved well, and she even tipped me. I was pretty surprised by that, tbh. It was still kind of awkward for me though.