401(k) employer contribution made in error

[Guest TinaThornton]

What does an employer need to do (if anything) if an employer contribution is mistakenly made to a participant's 401(k) account. Several employees were laid off and received severance checks (not "comp" under 401(k) plan definition). However, employer contributions were made based on the severance amount. I cannot find anything on this in the IRS correction procedures. Is this a windfall for the participants? Thanks.