The first was in college. A buddy of mine scribbled a facsimile of a $20 onto a piece of paper with a green marker. He then handed it to the checkout clerk at the cafeteria who took it and started to hand them back change. He stopped her and said "no, no it's a joke - look at what I just handed you". She was embarrassed but they both laughed together.
The second story which does involve the Secret Service is when my friend had a bunch of presents that he had wrapped and put in his front porch until was going to depart for a party. One of the presents was wrapped in a sheet of uncut dollar bills - which you could buy for that purpose.
A neppy neighbor saw it through the window and called the police who called the FBI who called the Secret Service who came knocking on his door to investigate. They were also embarrassed but I don't think they laughed. My friend told him he understands that they're just doing their job and that it's an important one.
I remember my friend coming home from his first year in college and telling me about how he passed a counterfeit $30 he'd found to a clueless clerk and they actually made the correct change. My wise-ass response was that that wasn't actually counterfeit, it was just fraud.