According to a blurb on CNN/Money, the IRS is sitting on a little more than 99k undelivered tax refund checks totaling $153M. The checks in question, worth an average of $1547 each, were returned to the IRS due to mailing address errors. As big as these numbers appear, these checks correspond to just 0.01% of all returns, so only a (very) small minority of refunds were affected.
Regardless, if you want to avoid problems, you should opt for direct deposit when you file your tax return. And if your check doesn’t show up, be sure to follow up with the IRS.
Speaking of which… If you were supposed to receive a check, but it never arrived, you can use the “Where’s My Refund?” tool to resolve any delivery problems.