Basically, the liability would result from one of two sources. First, taxable contributions to plans are considered wages, on which the employer is liable for withholding. Even if the employer has not in fact withheld, it is liable for paying the taxes which should have been withheld. The most common IRS action in the event of an audit is to go after the employer for those taxes, as opposed to trying to go after individual employees for income taxes.
The second possibility is that if the IRS decides to go after the employees, they might sue the employer for failure to disclose the problem. Of course, the success of such an argument would depend on such things as what statements the employer made to employees, etc.
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