Yesterday was cabbie Joseph Horvath's first day on the job, and he learned an important lesson: Never walk away with the keys in the ignition. Horvath had gotten out of his taxi around 12:45 p.m. to help his elderly passenger into a building at Fifth Avenue and 51st Street, across from St. Patrick's Cathedral. But seeing that Horvath was double parked, an unidentified doorman decided to "help" by jumping behind the wheel and moving the vehicle. That's when he clipped an oncoming Mercedes-Benz and careened onto the crowded sidewalk, where tourists were thronged because of Holy Thursday services. Luckily, he only grazed one pedestrian, a woman from Arkansas, who suffered a minor leg injury and refused medical treatment. Police are investigating the incident but have not charged the doorman yet. As for Horvath, he tells the Daily News, "It's embarrassing [and] I have to explain it to my garage. It's a good thing it wasn't my fault. I hope they don't give me the boot."