I tried to tip a cop once.
I will never forget once when I was about ten years old and riding with my grandfather (this was in the 1950s).
He was pulled over by a cop (in NYC). The cop asked for his license and registration, which he handed over. Then my GF did something unexpected. He said "I'm going to walk across the street to that newsstand and buy a paper. I'll hand you the paper when I get back.
A minute later, he returned to the car and handed the cop the newspaper. The cop examined it, put it under his arm, and handed my GF a "warning" (not a ticket).
As we pulled away, I said "What was that all about?"
My GF replied "Pretty normal thing. I put a $20 bill between the first two pages of the paper."