Yes, it's gonna be a problem.  The IRS gets copies of all W-2's and will discover that you understated your income.

ONLY AFTER the IRS has processed your income tax return and you've received your refund - assuming the IRS doesn't catch the error - then you need to amend your income tax return.

This FAQ

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

will explain the process.  You can't e-file an amended income tax return, you must mail it in.

Tom Young