It was the perfect New York hustle, a scam of subtle perfection. And for three years, it helped Mark Epperson pay his rent.

The hustle, in its simplest form: Borrow a Citi Bike. Ride it one block. Wait 15 minutes, then ride it back.

Earn $6,000 a month (under ideal conditions, and with lots of work).

“This is one of my side hustles,” said Mr. Epperson, an actor who lives on the Upper West Side and works as an understudy in “Perfect Crime,” an Off Broadway play. “I’m probably a vulture in some people’s eyes. And I guess that’s fair.”

If Mr. Epperson’s insight into the scheme is simple, the circumstances surrounding it are anything but. Citi Bike, a bike-sharing program operated by Lyft that offers 27,000 bikes throughout New York City, Hoboken and Jersey City, promises seamless pickup and drop-off. Occasionally, though, a ride to work ends with the rider’s discovery that the docking station nearest the office is full. A dash to brunch is foiled by an empty dock, with no bikes available.

Both situations are annoying, especially for Citi Bike subscribers, who now pay $220 a year. To fix the imbalance, Citi Bike uses various tactics to move bikes to in-demand stations. One involves hiring workers to drive panel trucks around the city, delivering bikes where they’re needed.

Another, created in 2016, is a program called Bike Angels, in which Citi Bike users move bikes in exchange for points that could be cashed in for swag like water bottles and backpacks, membership discounts and gift cards. This being New York, where even charitable activities quickly develop overtones of competition, a handful of users started racing to see who could win the most points. Citi Bike called them Power Angels.