Um... don't try to remove the power supply cover without turning off and unplugging it. 
Short form: The fan in the power supply went out and I replaced it. Put everything back together, switched it on, realized I'd put the fan in backwards. Shut it down, opened the case, and went to to open the power supply. Huge arc of electricity (thank goodness for insulated screwdrivers), totally fried the power supply. No other damage, thankfully.
^-.-^

Short form: The fan in the power supply went out and I replaced it. Put everything back together, switched it on, realized I'd put the fan in backwards. Shut it down, opened the case, and went to to open the power supply. Huge arc of electricity (thank goodness for insulated screwdrivers), totally fried the power supply. No other damage, thankfully.
^-.-^
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