I met so many clients: in the street, shops, banks etc.
Unless the client speaks to me, I simply nod politely and continue my way.

But a story was in particular funny.
One of my colleagues from the time I studied accounting made an appointment once.
I had no idea who the guy was and when I was about to buzz him in my building, I have recognised him straight away (had video intercom at the time).

I told him on the phone: "listen, we do know each other, are you aware of that? If you want, you can leave and I would understand this". He was actually brave and insisted he wants to come in. He visited me many times since then and we always had a good laugh remembering the past.