It will lead to you receiving more or less refund than you are entitled to depending on if you entered too much withholding or too little.
If you have already filed your return with an incorrect amount of withholding, then you will need to amend your return after your first return has been processed (you should wait until the first return is processed and any refund due is paid because its possible that the IRS may catch this mistake on their own and adjust your refund amount automatically)
It will lead to you receiving more or less refund than you are entitled to depending on if you entered too much withholding or too little.
If you have already filed your return with an incorrect amount of withholding, then you will need to amend your return after your first return has been processed (you should wait until the first return is processed and any refund due is paid because its possible that the IRS may catch this mistake on their own and adjust your refund amount automatically)
So. If I know I messed up, I should wait for the IRS to tell me I did before I jump the gun and amend?