When Oz is getting in the taxi, he calls at the end to take him and Jimmy to the Museum, and he gets in the front seat. In the next scene, he is in the back seat. Why this is such a big deal is because Jimmy's friend (the big guy) is the driver, and if he got in the front seat, he would have seen him.
This slip-up was not in Three to Tango. It was in The Whole Nine Yards. And besides, that didn't even happen! Oz didn't get into the front seat. He and Jimmy got into the back seat at the same time and tells the driver to take them to the museum.