When Porky and Gabby clock in at 10:15, there are no cards in the 'Out' section for them to punch in, and, in fact, Porky merely pulls the clock handle down (and Gabby tries to pull it down) without inserting a time card. If their time cards had been in the 'Out' section, where they should have been, management would have instantly known that they were late, so being caught sneaking in late would have been irrelevant.
The third time Porky throws a shoe at the meowing cat, he crouches down under the window. While crouched down, he lifts his head to peek out over the sill. At that point, his left foot flashes twice, indicating some cels that were not fully finished, or that his foot was just forgotten about when those cels were drawn.