There are often differences in the way that things are treated under tax accounting and public corporate accounting rules.

FORBES: Facebook Doesn't Pay Taxes On Its Profits: So What?

In 1999 the U.S. Treasury shut down a corporate tax shelter, promoted by accounting firm PricewaterhouseCoopers, known as the Bond and Option Sales Strategy, or BOSS. But BOSS didn't die.

FORBES: Hide the Baby

Experts say other whistleblowers are waiting for what they hope are imminent payments on large issues as varied as payroll tax withholding, tax shelters, illicit foreign accounts or arcane corporate accounting matters like transfer pricing.

WSJ: Taxes: How To Turn In Your Neighbor to the IRS