Not that I want to try this, but I was curious on what happened if you loaded a new magazine with the trigger pulled (both with slide open and closed).
Good question.
You can lock the slide back. Insert a loaded magazine. Pull the trigger rearward. Drop the slide and nothing will happen!
BTW: This is the same thing that happens each time you pull the trigger. The gun will only fire after you allow the trigger to move forward and reset.