This past century, large portions of the American South have been consumed by an unintended side-effect.
I refer to the infamous “kudzu” vine, imported from Japan for its stunning erosion-control properties. This tactic worked: Crumbling riverbanks and hillsides were stabilized all over the South—but, then, everyone realized with mounting horror that this hardy legume is almost completely unstoppable anywhere protected from hard freezes, growing at the awesome rate of a foot per night and reaching heights of 100 feet. Entire abandoned houses have been observed to vanish under a kudzu carpet, over a summer growing season. It's now considered a pest; sometimes, an outright menace.
The plant can be used with caution, where its invasive side-effect is known and planned for—but the point is that it was deployed without understanding its full effects.
Laws' side-effects, likewise, can render them self-defeating (though, on the bright side, bad laws are easier to eradicate than kudzu): I'm going to explain, below, why recently popular marriage-definition legislation like California's November 2008 “California Marriage Protection Act” (an initiative state-constitutional amendment) creates kudzu-class unintended side effects its proponents haven't anticipated and will find horrific—in that they're going to end up mandating and legally sanctifying exactly the sort of same-sex marriages they're intending to ban.
The article outlines eleven instances where the “visual” gender doesn't match the actual “genetic” gender and thus, the laws outlawing same-sex marriages could force “visual same-sex marriages.” Way to go, narrow-minded legislators!