When Doug and Richie sit on the couch waiting an unusually long time for Lynn and Carrie to come up, Doug says to Richie..."You gave them the apartment number, right?" The girls had just buzzed them from outside the building. The intercom at the building's front door is not connected to all of the apartments. They girls would have had to press the button for the guys apartment, meaning they would have already known the apartment number.
When Doug and Carrie are both in the shower in the beginning, the shower liner is hanging outside of the tub. This would mean that all the water splashing against it would run onto the floor.
When Doug is riding in the convertible at the end, there is only 1 sticker on his windshield ( there should be 2 ) and that sticker doesn't look like a NY motor vehicle sticker.
When the dog left Doug and Richie's apartment., it ends up outside. Being it was in an apartment building, it would have had to open 2 more doors in the lobby to get outside (the dog obviously exited the building when someone was entering or exiting).