Amended returns happen. The IRS is preparing for it:
As of April 18, almost 46 million returns were e-filed from home computers, more than the total from home computers for all of 2013. The IRS has received more than 131 million returns, of which 88 percent were e-filed.
The IRS also projects that almost five million taxpayers will amend their returns by filing Form 1040X during 2014. Taxpayers who need to amend their returns should file this form only after filing the original return. Generally, for a credit or refund, taxpayers must file Form 1040X within three years, including extensions, after the date they filed their original return or within two years after the date they paid the tax, whichever is later. For most people, this means that returns for tax year 2011 or later can still be amended.
We know of at least one return that will be amended this year...