See IRS IRB 2007-22. There is no 10% penalty on excess employee contributions. The "penalty" is that the excess Roth contribution and attributable earnings will be taxable when eventually distributed even though the Roth contribution was not excludible from income, resulting in double taxation of the excess and taxation of the attributable earnings even if the distribution would otherwise be a qualified distribution. The excess and the attributable earnings will be the first amounts distributed when you do take a distribution from the Roth 401(k) and that amount will not be eligible for rollover.
