aeroplane rule
airplane rule
"Complexity increases the possibility of failure; a twin-engine aeroplane has twice as many engine problems as a single-engine aeroplane."
By analogy, in both software and electronics, the implication is that simplicity increases robustness and that the right way to build reliable systems is to put all your eggs in one basket, after making sure that you've built a really *good* basket.
While simplicity is a useful design goal, and twin-engine aeroplanes do have twice as many engine problems, the analogy is almost entirely bogus. Commercial passenger aircraft are required to have at least two engines (on different wings or nacelles) so that the aeroplane can land safely if one engine fails. As Albert Einstein said, "Everything should be made as simple as possible, but not simpler".
See also KISS Principle.