Accidentally claimed exempt all year what will happen

[Critter]

Well the year is not yet over so get that corrected ASAP ... do Single & zero allowances at least for the rest of the year ... you may even want to add an amount on line 6 of the W-4  if needed.

If you don't pay in enough thru the payroll system and/or estimated payments then you can be charged penalties & interest on the underpayment.