When Jack drives Rebecca home, the car is wet from the recent rain. However, there is no sign of rain on the front windshield, even no windshield wiper marks.
The balloons seen in the background at the carnival were attached to ribbons and with knotted necks. In 1972 helium balloons, particularly those sold at carnivals, were attached with nylon strings, not ribbons, and were plugged, not tied.
The Immaculate Reception, which served as a backdrop, occurred on December 23, just two days before Christmas. However when Jack and Rebecca first meet at the bar, and in the subsequent date at the carnival, there are no indications of being during the Christmas season (decorations, music, etc). In addition, it is unlikely such a carnival would occur during the Christmas season, and the thunder accompanying the rain storm is inconsistent with winter weather in the Pittsburgh area.