About 30,000 active-duty troops and a similar number of Reserve Component members owe the Internal Revenue Service a collective $390 million in back taxes.   Beyond the normal errors that many filers make, experts say service members face unique challenges in sorting out their taxes.  One common issue is the combat tax exclusion, which allows troops to avoid paying income tax for any month in which they serve in designated combat zones.  It can be confusing because it’s hard to know what to include and what not to include. Another factor is frequent changes of address as deployed members move to new assignments over their careers. Tax documents mailed to the wrong address can lead service members to mistakenly file incomplete returns — a red flag for the IRS.  And if the IRS finds a problem with a return, some troops who have recently moved might miss the notification letter warning them about possible delinquency. Taxpayers who fail to respond to IRS delinquency notifications can miss an opportunity to straighten out the problem before it becomes bureaucratically set in stone. For service members found delinquent, the IRS will likely enforce a levy on paychecks until full payment is received, the same as for civilians and private-sector workers who run afoul of the IRS.  The IRS delinquency rate for service members, both active and reserve, is about 2 percent, slightly lower than the rate for federal civilian employees, which averages nearly 3 percent.  The delinquency rate for military retirees is higher. Nearly 4 percent owe a total of $1.5 billion in back taxes.