MURPHY'S LAWS FOR ENGINEERS

• Any wire or tube cut to length will be too short.
• After any machine or instrument has been fully assembled, extra components will be found on the bench.
• Any error that can creep in, will. It will be in the direction that will do the most damage.
• All constants are variable.
• In any given miscalculation, the fault will never be placed if more that one person is involved.
• Dimensions will always be expressed in the least usable terms.
• The probability of a dimension being omitted is directly proportional to its importance
• It is impossible to make anything foolproof because fools are too ingenious.

From http://rfcafe.com/miscellany/humor/murphys-law-for-engineers.htm