1. All warranty and guarantee clauses become void upon payment of invoice.
2. Dimensions will often be expressed in the least usable terms. Velocity, for example, will be expressed in furlongs per fortnight.
3. Identical units tested under identical conditions will not be identical in the field.
4. A dropped tool will land where it can do the most damage. ( Also known of the law of selective gravitation.)
5. The probability of a dimension being omitted from a plan or drawing is directly proportional to its importance.
6. Interchangeable parts won't.
7. Probability of failure of a component, assembly, subsystem or system is inversely proportional to ease of repair or replacement.
8. If a circuit cannot fail, it will.
9. A fail-safe circuit will destroy others.
10. A transistor protected by a fast-acting fuse will protect the fuse by blowing first.
11. A failure will not appear till a unit has passed final inspection.
12. Manufacturers' spec sheets will be incorrect by a factor of 0.5 to 2.0, depending on which multiplier gives the most optimistic value. For sales persons' claims these factors will be 0.1 to 10.0.
