Aaron Wall announced today that SEOBook is temporarily closing to new members. This means that if you're an affiliate of his, you won't get any commission for the 2 months (or more) that they're doing this software upgrade or whatever.
This is one of the problems with affiliate programs - you're at the mercy of the merchant. I've had cases where merchants kept constantly changing the ads and banners, where they kept revising their terms every few weeks and you had to read detailed legalese on a frequent basis (unless you're the type who just trusts everyone and clicks Accept), where the pricing structure, commissions and incentives kept changing ..... it's a pain. The smaller the program earnings for you the more of a nuisance all these little changes are.
What's your bug bear with merchants?