It's an old-school rule that most people seem to have forgotten: If someone's going heavy, don't get between them and the mirror.
The mirror is sacred. The heavier the weight, the more sacred it gets. Not for vanity's sake, but as a tool. For many lifts, your form is going to be more controllable if you can see what you're doing. Seasoned lifters know this and that's why we stay out of the way of someone doing a heavy set in the mirror. For anyone else, that means don't get between them and the mirror!
If you want a pair of dumbbells and they're right in front of a guy doing seated shoulder presses with a pair of 90's, you wait. If you have a pair you actually want to put away, right in front of him, that's right, you wait. That's what he'd do for you. Doesn't matter who's bigger or how much weight is involved.
And God help you if you block someone's mirror to take a selfie. Breaking this rule can definitely, and certainly has, ended with hands being thrown.
Also, don't be the guy who grabs a pair of dumbbells and does his set three inches in front of the rack. There's a two-foot-no-man's-land in front of the mirror. Only cross that path to grab or put away your dumbbells and get out.