Mirrors are not 100% efficient. That fraction of one percent will kill you if the incident power density is high enough. Once one little portion of the mirror i

Mirrors are not 100% efficient. That fraction of one percent will kill you if the incident power density is high enough. Once one little portion of the mirror is absorbing more energy than it can dissipate, that portion will heat up and up. When it gets hot enough it tends to become less reflective - it oxidizes or maybe just evaporates - and then you've got even higher absorption at that spot, which makes it fail even faster and helps heat up adjacent spots... When this happens with a pulsed laser you can sometimes hear it. That tiny snap-snap-snap is the sound of your three-hundred-dollar optic heading for the trash. You'll look at it and see the telltale little spots. If you're cheap you'll try just sliding it sideways to a non-spotted bit of surface, but sometimes there isn't enough mirror for that... That's with a high-tech custom-designed laser mirror, not a random surface of some random object. And we're generally not trying to destroy it. And yet they often get destroyed, when you screw around with high-intensity lasers.