10
The winners list, available on request
The clause that lets you check whether anybody actually won.
Near the end, usually in the smallest type on the page: send a stamped self-addressed envelope to this address after such-and-such a date and we will send you the list of winners.
Almost nobody does it. It is nonetheless the mechanism by which the central promise of a promotion is falsifiable. Prizes were advertised; the rules said all prizes would be awarded; here is the list of who got them.
Modern promotions increasingly publish it on a web page instead, which is better in every way except one: a page can be edited and a posted letter cannot. If you are ever genuinely curious whether a promotion pays out, this clause is how you find out, and it costs a stamp.
That is the ten. Not a long list, and between them they tell you who is running the promotion, whether it is lawful, what you can actually win, how likely that is, what happens if you do, and how to check they were telling the truth. Ninety seconds well spent.