Comment by mrob

From Wikipedia's "Electrolytic capacitor" article:

"Applying a reverse polarity voltage, or a voltage exceeding the maximum rated working voltage of as little as 1 or 1.5 volts, can destroy the dielectric and thus the capacitor. The failure of electrolytic capacitors can be hazardous, resulting in an explosion or fire."

Even a small one could take somebody's eye out if they happened to be close to it when it failed and not wearing eye protection.