I was put in charge of switching out the 40 W laser used by the Extreme UltraViolet Lithography Project and replacing it with a monstrous 1500 W laser system- 3 beams, each 500W. The wavelength was 1064nm, so completely invisible to the human eye, but the eye could still focus it on the retina. Your first warning would be an audible 'pop' as a section of your retina vaporized and then your eyeball would fill with blood.

In addition to warning signs, audible alarms and flashing warning lights I took a pair of broken laser goggles and placed them in the beam for a second and hung the resulting mess on the door to enter the lab- it was an effective warning. 1 second of exposure was enough blow a hole through the goggles. I also posted a first hand account from someone that had eye damage from a similar less powerful laser- it included the line "It was like looking at the world through a fishbowl filled with glycerin, blood and black pepper flakes"

:) People took laser safety pretty seriously when they came through that door.

That laser scared the crap out of me... part of the alignment process was using a flashlight to look for smoke from stray light hitting something it shouldn't...