A tricky rule keeps tripping up thousands of Canadians who make withdrawals from their tax-free savings accounts, and replace the money too early.

Some 54,700 taxpayers got warning packages from the Canada Revenue Agency earlier this year about the problem affecting the 2013 taxation year, and were told they face a penalty.

The number has been dropping steadily from a peak of 103,000 in 2010, but still represents a persistent misunderstanding of TFSA rules even as the agency and financial institutions step up education measures.

The regulations say that account holders can put back the amounts they withdraw from a TFSA only in a later calendar year. Doing so in the same calendar year exposes them to a tax hit for overcontributions, even though they're only replacing the withdrawn funds.

By the end of 2013, some 10.7 million Canadians had opened a TFSA, a savings vehicle introduced by the Conservative government in 2009 that allows money to grow inside tax-free with no income-tax hit on withdrawal.

The popular savings tool cost the federal treasury some $410 million in forgone taxes in 2013, or more than a billion dollars over its first five years.