Forbes has a slideshow of what they consider to be the eight worst financial laws that have been enacted. As a companion piece, they've also highlighted three reasons why financial regulations never go as well as they idealistically should. One of their (rather depressing) observations:
No matter what the rules are, the financial industry will figure out how to innovate around them. "New regulations often let people find ways within the letter of the law to do whatever they wanted to do in the first place," says Edward Kane, a professor of finance at Boston College. To wit: After the Enron scandal, the Financial Accounting Standards Board drafted rules intended to make companies confess to dicey assets held in separate entities that weren't consolidated on their balance sheets for all eyes to clearly see. Those defenses didn't exactly hold (see "End-Run Debt").
I won't list out all the financial laws they go through -- but I'm pretty sure you'll recognize number 7.