Wall Street Journal: 
How to Owe Capital-Gains Taxes Without Even Trying:

The byzantine U.S. tax code doesn't merely defy logic and comprehension. It also keeps producing ways to trip up investors. …

In "corporate inversions," … a U.S. company
creates a new parent incorporated in a foreign country. The old shares
in the U.S.-based company are then exchanged for stock in the foreign
parent—a swap that the IRS regards as a taxable
transaction. There were six such deals in 2011 and 2012, says Omri
Marian, a tax expert at the University of Florida's Levin College of
Law. More could be in the works, say tax analysts. … 

But the question remains: How can shareholders be put into the
bizarre position of owing taxes upfront on shares they never sold?

After six companies in the S&P 500 moved offshore between 1999
and 2003 to benefit from lower corporate tax rates, Congress and the IRS
cracked down. Complex regulations made it harder for U.S. firms to
expatriate. Under those rules, shareholders who purchased the U.S. stock
at lower prices incur capital gains when it converts to a foreign
company.

While fewer firms have moved abroad, the U.S. tax code remains unfair
in the eyes of many corporate executives and analysts—since American
and foreign companies often aren't taxed at the same rates. "We should
not be harder on our own children than we are on the children of
others," says Bret Wells, a tax-law expert at the University of Houston.