A comment someone made a couple weeks ago started me thinking about common rules of thumb that engineers follow. Here's a few that I've seen in practice.
If something can go wrong, it will. When I was in high school I thought this was clever and sarcastic way of expressing a dim view of the universe. It wasn't until I was an engineer that I began to appreciate the statement as an elegant way of describing statistical inevitability: if there is a finite chance that something will happen, given long enough time it WILL happen.
Engineers commonly double the amount of data they take in the name of Nyquist sampling. It may not be necessary, but that's the way it works.
Once is an anomaly, twice is a coincidence, three is a pattern
This is something that I've said for many years, just as a general rule for test engineering. When I did a search for it, the most common mentions of this are either from the novel Goldfinger or The Moscow Rules (which quotes Ian Fleming). But I think it is most applicable when you're debugging something.
Rinse it six times to clean it
There are well over a thousand references to this on Google. Rinse something out with water six times to clean it. It's a simple rule of thumb.
Double the safety factor
My first job after grad school was at an aerospace engineering firm. There were a lot of mechanical engineers there, and I quickly learned of the prevalence of safety factors from them. It was an unspoken rule that, whatever safety factor you calculated, before you finished the design that factor had to be doubled. Better safe than sorry.