About a half hour into the film, Rocky is looking at pictures and clippings on the wall, and there is the sound of a steam locomotive coming up to speed, along with the sound of its steam whistle. Diesels had long since supplanted steam in Philadelphia and most of the country in 1976.
During the big fight, the arena is clearly empty at times. This is because only 50 extras could be afforded for the scenes, and despite cinematographer James Crabe's best efforts to hide this fact, occasional, if one looks hard enough, one can see most of the arena is empty.
When Rocky (Sylvester Stallone) is knocked down during the fight, and on the verge of defeat, you can audibly hear Rocky's manager and trainer, Mickey (Burgess Meredith's voice on an ADR dub) yelling to Rocky, "Stay down!" as he does not want to see Rocky take a further beating. However, you can see Rocky's corner man, Mike (Jimmy Gambina), mouthing "Get up!"
At the end of the fight, when Apollo and Rocky are hugging, Apollo's manager comes up to him in the close-up shot. Then they switch to an aerial shot, and Apollo's manager jumps over the ropes into the ring.
When Adrian enters Rocky's apartment for the first time, Rocky is wearing fingerless gloves when he removes his hat and sweater. In the very next shot when he is showing Adrian the turtles he bought, he has no gloves on.
When Rocky enters the bathroom while Mickey is visiting him, three darts are on the door. After Rocky opens the door the first time, the darts are in a completely different pattern. In the final shot, the door has 2 darts.
Rocky wakes up at 4:30am to start his exercise regime,it is December. When we see him out jogging up the steps,the dawn is breaking. The sun should not be coming up until after 7:00am (Unless he ran for 2 & 1/2 hours!)
The closing credits list real-life boxing referee Lou Fillipo, who plays the referee in the Championship fight, as "Championship Fight Announcer". Thayer David, who plays the promoter Jergens, was the fight announcer.
Rocky (who is supposed to be left handed) uses right hand to write down info on collections he is supposed to make in two scenes and to turn dial on combination lock at beginning of movie. However, just because Rocky boxes Southpaw does not mean he is necessarily left-handed.
When Adrian and Rocky leave for their date, there is a rose bush in front of the house that has two fresh roses on it and some flowers on the ground in bloom. Since it is late November in Philadelphia and it is obviously cold, this wouldn't be possible.