I'm fascinated by the puzzle of how you can know that you don't know that you don't know.
The only rule of thumb I've came up with so far is that human activities are COMPLICATED. If people have been doing it for more than a week, then there is more to it than appears at first glance. If there exist people with a recognized expertise, then seek them out -- particularly before you do anything that could get you hurt.
But that just brings us up to the same problem. I was just having a conversation on some prop-making forums about CO2 effects. It seems very simple. Take CO2 tanks and valves, which work perfectly well when used as designed, but don't produce that nice white smoke. Turn the tank upside down so it siphons. Now the same tank and valve produce that lovely "fire extinguisher" cloud. And at some random moment, a valve or bit of line shatters under 500x expansion and gives you a serious injury!
Since I work in theater, I am frequently around people who are dealing with electricity, rigging, but even simple levers in ways they haven't realized could cause injury. What they don't know they don't know can still hurt them. They don't know enough to know when they've left the domain of "My knowledge is sufficient/my ignorance is harmless" and entered the domain of "This could kill someone but I don't even realize it is dangerous."