What hurts a speaker?

Rosstaman makes excellent points! You should also never mention in front of a speaker that it lacks bass or does not image well. Do not compare monitors to full range speakers - in extreme cases they have been known to cry. Also (very important) NEVER mention - even in passing - that you would like to try an electrostatic speaker when in earshot of a dynamic speaker or vice-versa. This is extremely gauche behaviour!
Really offended speakers have been known to get even by eating the output devices of your favorite power amp or intentionally smearing vocals and string instruments. Speakers are also capable in extreme cases of transmitting subliminal messages to new/prospective dating candidates - Ya don't want your speakers hinting to the new girlfriend that your have crabs, DO YA?

Be nice to your speakers and they will in turn be nice to you.

A recent and real story: Fired up the amp only to jump backwards at a deafening and startling sound. The 1.5 yr. old had adjusted the volume (to the top) and switched the tuner on. The woofer seals both split. Fortunately, it was the cheap (relatively) basement system!

Esteemed gentlepeople. What has always hurt any speaker most is no one listening (OK, horns excepted). Otherwise, pandering to the turntable is especially egregious.

Getting into an argument with the girlfriend over the phone, losing your tempter and throwing the phone....
through the speaker..

Also on the list..
Shotguns
Pistols
Flame Throwers
Falls from any great height
Acid
Gnomes
N'Sync
Spitting Llamas
heroine needles.

Rap Music!!!!! :)