Have you ever dealt with this as a payroll professional? What did you do?
If it was truly invalid, I would explain why and ask the employee to complete a new one. When you say questionable, what do you mean? If they're just claiming more exemptions that you think they should or are able to, As long as it is valid in every other way, I would still accept it as you don't truly know what their situation is outside of office.
Agreed. The W-4 is valid or it is not. There is no middle ground. No such thing as a "questionable" W-4. An invalid W-4 is 100% dead.
After 30+ years in the payroll business, had a new one, an employee wanting me to withhold 25% of their gross for SS. Hand written on their W4...
That should not be written on the W-4 as such - two problems, percentages are not permitted and the withholding requested on the W-4 is for income tax, not Social Security. The addition to the W-4 is an unauthorized one and that would make the W-4 invalid. Otherwise, requesting additional withholding in a specific dollar amount that just happens to equal the amount necessary to get to the amount desired (25% of gross for SS) is OK - no reason necessary. My guess is that the employee's tax professional suggested that percent to cover something like - hmmm - I don't know - Self Employment tax? That sounds like Social Security to many people.
Usually problems with invalid W-4s are about avoiding withholding. This one goes the other way. Ask the employee to submit a new W-4. And maybe suggest that the tax professional help prepare one that is valid that accomplishes what they want done.