Some one else pointed out that 'mask on forehead' is an indicator of a problem
A lot of what we are taught is you look for indicators of a problem, during and after a dive. [1]
(failure to respond to signals, looking up towards the surface etc.)
As such, some actions are considered bad practice. Leaving cylinders standing upright, I was taught always to lie cylinders down, or tie them to the rail (on a boat).
Personally, I put masks on foreheads as bad practice. Which I think is one of the reasons that posed pictures (head shots) of divers tend to show the mask around the neck, or on the back of the head. You very seldom see a picture of a diver with the mask on the forehead.
I don't know how many of you have seen video of people in difficulty in public swimming pools. It is often very difficult to identify the person in trouble. Lifeguards are taught to look for specific indicators. Lack of an indicator does not mean someone is ok, similarly, an indicator does not mean they are in trouble, it is a cumulative information that helps the lifeguard.
There are no dive police. You will not receive a fine, you will not loose your dive card. HOWEVER, there are people I won't dive with, or at least avoid if possible, there are a number of reasons for this, from personality, behaviour, safety, etc.
Some I am wary of, having only just met them on the boat. There is a wonderful phrase
"All the gear - no idea". Thats what instructors and DM look for when a new group gets on the boat!
[1] - I know some agencies wait until you are an instructor or rescue diver before they focus on this. I was taught very early on, and each subsequent qualification, so its ingrained.