Most subway riders can quickly navigate through a revolving-door, ceiling-to-floor turnstile. It's a simple enough maneuver: Swipe the MetroCard, step straight ahead, push the bars forward.

In less time than it takes a conductor to say, "We're being held in the station by the train dispatcher," the seasoned straphanger has crossed to the other side.

But the cagelike contraptions can bamboozle less experienced travelers into paying the $2.25 fare twice - an apparently unintended consequence of the design that doesn't seem to trouble the Metropolitan Transportation Authority.

"These are the biggest robbers down here," one transit worker said as he repaired a high entrance-exit turnstile, or HEET, in the Union Square station.

Here's what happens. A confused or distracted rider steps into the wrong opening of a revolving-door turnstile after paying. From that position, slightly left of center, the turnstile will rotate in the correct, counterclockwise direction - for a few feet. It%A0then locks again.

The rider is still on the outside looking in. The turnstile won't budge unless fed another $2.25.

"It happens all the time," another turnstile repairman said. "Most of the time it's tourists, but sometimes it even happens to people who live here. Nobody knows. There's no signs or information."

Next to tourists, the elderly are most likely to need more than one crack at a HEET, the workers said.

Sara, a Westchester mom, and her daughter Ellie got tripped up the other day at the Times Square complex. As they approached a pair of turnstiles, dozens of exitingriders from a 42nd St. shuttle train pushed through from the other side.

The tiny entranceway suddenly was crowded and noisy. A screaming, brain-rattling alarm sounded because a self-absorbed joker used the emergency exit gate for no goodreason.

Mother and daughter, a bit bewildered and trying to advance upstream through exiting riders, both pushed the wrong set of bars and each lost...