The scene has one firefighter in the water and one standing on the platform. Suddenly there are two firefighters in the water. Next there are two firefighters on the platform, one wet and one dry.
In the beginning where Severide dived on the sunken car by himself. In reality, a rescue diver would never go down by themselves for safety reasons as shown when his rope was caught and had to be cut. Two or more divers are required for just such an issue.
When Otis is explaining completing the Saltine Challenge, he's supposedly live-streaming the event online. However, multiple shots of his laptop screen show the same still image and not a video feed.