We had Sunday brunch at Maison Kayser. It's a semi upscale French bakery and café, moderately expensive but not over the top. Brunch for two $30-40 before tip.
The late 20's couple sitting next to us spoke French, I assumed they were French but it's not the point.
They finished their meal before us and gave the server a $100 bill with the check. The server returned with their change and I noticed that he left $1. for a tip.
I know the service was exceptional and could not bite my tongue. I caught his eye and said unbelievingly: "One dollar??, servers here work for tips."
His response: "I am a poor student." I saw designer jeans and running shoes and said: "If you are a poor student you should eat at McDonalds or a diner."
He grumped, looked down at the dollar and left 5 more.
When I travel I do a minimum of research to find the local customs regarding tipping and such. This obviously well educated couple should have known better.
Thanks for letting me share.