If you shoot at a slow shutter angle, you may end up with motion blur in front of the green screen.
This is something that is really hard to fix in post production since the areas where the motion blur occurred will be blended together with the green screen. It can be hard or even impossible to set up a clean chroma key without losing parts of your subject.
Try to have your subject move slowly to avoid motion blur to begin with.If you need fast movement, use a higher shutter speed (a smaller shutter angle) to avoid motion blur. Because this will impose a certain hypersensitive onto your footage, you can add the motion blur back in post production - AFTER you applied the green screen
You shot in front of your green screen and the subject was out of the screen for just a short moment. Not a problem, right? Fix it in post?
This can be very annoying to fix as you will have to manually rotoscope out all the elements that left the green screen.
While the rotobrush tool in After Effects may help make the job easier, I strongly recommend reshooting the footage if you can
This is not really a problem, more a tip on how to make your subject stand out a little cleaner against the green screen. This will make application of the chroma key a lot easier and it will help give the actor or object a bit more shape by lighting them smartly
If you just shoot your subject against a green screen with light from the front, the outline of the subject is not very well defined.
Add a light between the green screen and your subject but aim it towards the camera at the back of your subject. This will create a nice highlight outline around your subject and give it shape and make it stand out nicely against the background