If you thought paying taxes in one state was hard enough, try completing 18 different tax returns.

That's what LeBron James had to do for 2014, as the Cleveland Cavaliers' superstar and all of his fellow pro athletes are taxed in many of the states where they play games.

Tom Brady, according to For The Win, had to file tax returns in at least nine states. That's because whenever he practiced or played a game in another state and thus earned a salary there, he was technically making taxable income.

This practice is linked to Michael Jordan and his Chicago Bulls teammates. After the team won the first championship of its dynasty, over the Los Angeles Lakers in 1991, the state of California moved to tax the Bulls players who had earned income while playing in Los Angeles. This infuriated the state of Illinois, which created its own laws taxing visiting professional athletes. The legislation in Illinois came to be known as "Michael Jordan's revenge."

Now, more than two decades later, a host of states tax players for working in one of their cities. Brady, for example, was taxed in Arizona after only playing one game there. It was a big game, of course, and Brady reaped enormous benefits for winning.

As one might expect, athletes aren't happy about this practice. Especially because in some states, like Ohio, most professionals who spend fewer than 12 days earning income in that state are not taxed.

“It’s not even the financial burden," Stephen Kidder, one of the country's leading sports tax lawyers, told the Boston Business Journal. "It’s the fact they’re being treated in a discriminatory way, compared to all other taxpayers."

Most players hire an accountant to handle their messy tax returns. These men and women, like K. Sean Packard of OFS Wealth outside Washington, D.C., have their work cut out for them.

"I had a guy who played for four teams in 2014," Packard told USA Today. "That return was nuts."

The most unusual tax returns this year may have come from Derek Jeter, who earned all sorts of expensive gifts in other states.

The best situation for a pro athlete, in terms of both time any money, is to play in a state like Florida or Texas that does not tax income.

More: Thanks To All Of His Farewell Presents, Derek Jeter Will Have A Hefty Tax Bill

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