...is that you have to break a couple of 'em at once to put a hole in anything important. And, yes, manually de-cocking a single-action weapon is a Rule Three violation.
Sometimes people violate the Rules quite on purpose. Technically, every time I engage in dry-fire practice, I'm committing a Rule One violation, so I definitely avoid committing any Rule Two or Three violations at the same time. Carrying in a horizontal shoulder holster or a pocket holster is a Rule Two violation.
Violate the Four Rules at your peril. You'll notice Unc was careful to observe all the other Rules, and is therefore no worse for wear, save for ringing eardrums and a scuff to the pride. (Based on my personal experiences with other experienced shooters and unexpected loud noises, he's going to be a regular safety Nazi for a bit, too. This is not a bad thing at all...)
UPDATE: Other, very valid and educated, points are made here in the comments section.