The circuit designed to protect the unit from catastrophic failure won't.
The circuit designed to be highly reliable isn't.
Any circuit design must contain at least one part which is obsolete, two parts which are unobtainable and three parts which are still under development.
If x is the amount of time you wait to touch a hot vacuum tube, the time for the tube to be cool enough to touch is greater than x.

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing.
A complex system that works has invariably found to have evolved from a simple system that works.
Logic is a systematic method of coming to the wrong conclusion with confidence.
Any component removed, tested, and found good has a extraordinarily high chance of breaking in the re-installation process.
Build a system that even a fool can use and only a fool will want to use it.
Always draw your curves, then plot your readings.
Do not believe in miracles - rely on them.
Firmness of delivery dates is inversely proportional to the tightness of schedule.
Dimensions will always be expressed in the least usable term. For example, the velocity will be expressed in furlongs per fortnight.
Any wire cut to length will be too short.
Tolerances will be accumulated unidirectionally toward maximum difficulty to assemble.
A fail-safe circuit will destroy others.
A transistor protected by a fast-acting fuse will protect the fuse by blowing first.
Only after completely disassembling the unit will you realize the reason it wasn't working was because it wasn't plugged in.
Any error in calculation will be in the direction of most harm.
In specifications, Murphy's Law supersedes Ohm's Law.
If a safety factor is set through service experience at an ultimate value, an ingenious idiot will promptly calculate a method to exceed the safety factor.
Given any problems containing 'n' equations, there will always be 'n+1' unknowns.
Any sufficiently advanced technology is indistinguishable from magic.
If the number of screws which must be removed when disassembling a given piece of electronic device is represented by x, the the number of screws used when reassembling it will be some number less than x.