The current 401k contribution limit is $18,000. If I'm trying to contribute the maximum amount for the year and accidently go over, what happens? For example, if I end up contributing $20,000 for 2016 do I get a reimbursement of $2000 that's taxed at my income rate? Does the IRS enact any kind of penalty tax for exceeding the yearly limit?
EDIT: To summarize how contributing too much is possible - my current employer offers opportunities for bonuses. I've calculated out a flat rate to deduct from each paycheck that will max out my contribution when combined with the employer match. However, if I qualify for a bonus it's treated as another paycheck instead of being added to the current pay period. My 401k contribution is then deducted from both my regular paycheck and the bonus check. (Normal taxes would also apply to the bonus). And just for clarification, all deductions are pretax. This is a traditional 401k, not a roth.