Murphy's Laws of Engineering

Any wire or tube cut to length will be too short.

Identical units tested under identical conditions will not be identical in the field.

After any machine or instrument has been fully assembled, extra components will be found on the bench.

A dropped tool will land where it can do the most damage (also known as the law of selective gravitation).

Components that must not and cannot be assembled improperly will be.

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 computation, the figure that is most obviously correct will be the source of error.

The most logical way to assemble components will be the wrong way.

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.

After the last 26 mounting screws are removed from an access plate, it will be discovered that the wrong access plate has been removed.

The probability of a dimension being omitted from a set of instructions is directly proportional to its importance.

It is impossible to make anything foolproof because fools are so ingenious.