The failing most common among the untrained or the poorly trained (and the frightened) is to lean entirely too much on the white light.
Thankfully because most of the criminal types are not meat eating gunfighters and are more interested in escape than a body count the improper use of lights doesn't routinely have dire consequences.
Nevertheless, it's not good to plan on encountering just the stupid or the scared.
There is a balance to be reached in the use of white light. It's almost impossible to navigate (especially in unfamiliar surroundings) entirely in the dark and you're darn sure not going to be able to clear the room without using SOME white light. (Unless you have night vision)
It's also not a good idea to walk around with the light turned on all the time so you can see where you are going. The compromise is to use short bursts of light to allow you to navigate in the dark.
The bursts of light should be around ½ a second, which is just enough to allow you to see what you need to see… and you are moving the whole time so that if a bad guy does see the burst of light you aren't where it was. Some folks call this “flashing”, some call it “strobing”.
It's used to navigate and to see what you need to see inside a room. You can't simply enter a room that you haven't checked out at all… that's asking for disaster.
Generally you want to try and use the LIE principle in low light. LIE stands for:
½ second burst of light to locate a potential threat
1 second burst of light to determine whether or not the potential threat you located needs to be shot
Pulling the trigger, with the light off if possible. (It's not always possible)
After each of these steps you MOVE so as not to be in the same location as the light or the muzzle flash (Muzzle flash is also a target indicator)
Now this process if done at speed is extremely difficult for the guy on the other side of the light to figure out.