Any project will require at least two trips to the hardware store.
If you need more than one item (pair, four, etc) the probability that one will be damaged or the wrong color is directly proportional to the desire or need of the object.
You always need more paint
You never have enough nails, screws, glue, or other fasteners
The probability that you will complete a weekend project before the end of the weekend decreases with when you actually start the project. (see Law #6)

Corollary: Any plumbing project started after 4pm on Sunday will require an emergency call to the plumber to fix the broken pipe.
To estimate the amount of time needed to complete a project: estimate the amount of time needed, multiply by two and use the next highest unit. A one hour task will take two days to complete.