No asset allocation gameplan. Most people don't understand their investments. Asked why they invest the way they do, the vast majority of 401(k) participants don't know.
Chasing last year's best performers. The investment world is one of cycles. Take a look at the best to worst performers by asset class each year. No one asset class is always the best or worst place to be. Last years top performer is just as likely to be the bottom of next year's list.
Electing a percentage to be invested in a plan and never changing. If you're 40 today, you aren't the same person you were when you were 20. Why are you putting the same amount of money in your 401(k) plan? Time to re-examine what you can afford to save for retirement.
Taking a cash distribution from a plan upon termination. The most tragic of 401(k) mistakes. You pay a 10% penalty on the distribution if you are under age 59 ½. You also pay income taxes on that distribution. Most importantly, you rob yourself of what those dollars would have been worth at retirement.
Investing the same as the guy in the next office. Your goals and dreams are different from the other guy. Why not come up with a plan of attack all your own?
Make sure you don't make these mistakes too!