The 'No-Show' or Late Arrival
The cardinal sin. If you are booked, you must turn up. If you are late, you delay the entire unit base processing. Agencies have a low tolerance for this; one no-show can get you removed from their books permanently.
Looking at the Camera
It's a natural instinct, but it ruins the shot. You must ignore the camera (and the main actors) completely unless directed otherwise. 'Spiking the lens' forces expensive retakes and frustrates the director.
Posting on Social Media
Never, ever take photos on set or post about what you are filming until it is released. NDAs (Non-Disclosure Agreements) are standard. Leaking a costume reveal or a set location on Instagram can lead to legal action and an immediate ban from the industry.