Problem weeds appear first at the back end of the field you visit least. The number of salesmen that will call on you on any given day will be directly proportional to the amount and urgency of the work you have to get done. Weed escapes increase exponentially once the corn is too big to cultivate. Spray drift is selectively attracted to soybean fields. The last round to be plowed next to the line fence is always one furrow narrower than the plow.
Tires never go flat when the gravity wagon is empty.
Cattle never break out while you're watching. The announcement of the one meeting you most wanted to attend will arrive in the mail the day after the event. The day you let your cousin from the city fuel up your biggest tractor will be the day after the dealer put gasoline in the diesel tank. Now if the truth of all these laws makes you depressed, just remember that Murphy's Law even applies to itself. At times, it goes wrong too, allowing you to accomplish something.