The 5 Most Common 401(k) Plan Mistakes (and How to Avoid Them)

Even the most organized HR and payroll teams occasionally make 401(k) plan errors. With multiple systems, tight deadlines, and changing regulations, it’s easy for small details to slip through the cracks.

But here’s the good news: most 401(k) plan mistakes are avoidable with the right checks and communication in place. Recognizing what can go wrong (and building safeguards early) helps protect your employees and your fiduciary responsibilities.

Below are five of the most common 401(k) plan errors employers face, along with simple ways to help prevent them.

1. Missed or Incorrect Employee Deferrals

What Happens

An eligible employee isn’t enrolled on time, their deferral doesn’t start as scheduled, or the wrong percentage is deducted from their paycheck.

Why It Matters

Missing deferrals can create compliance problems and frustration for employees who wanted to save. It can also require corrective contributions (sometimes with additional earnings) to make participants whole.

Ways to Prevent It

• Use automatic enrollment reports to confirm all eligible employees are included.
• Double-check payroll integration after each system update or new hire.
• Keep a shared eligibility tracking spreadsheet for HR and payroll teams.
• Work with a 401(k) advisor to evaluate your options and strengthen your oversight process.

2. Late Deposits of Employee Contributions

What Happens

Employee deferrals are deducted from paychecks but not deposited into the 401(k) plan right away.

Why It Matters

The Department of Labor (DOL) requires deposits to be made as soon as administratively possible, typically within a few business days. Late deposits may count as prohibited transactions and require reporting and correction.

Ways to Prevent It

• Establish a clear deposit timeline and automate transfers when possible.
• Assign a backup staff member to handle deposits if the primary contact is unavailable.
• Reconcile contribution reports monthly to ensure timing accuracy.
• Work with a 401(k) advisor to evaluate your options and strengthen your oversight process.

3. Employer Match or Profit-Sharing Errors

What Happens

The employer match formula is applied incorrectly, or match contributions are based on the wrong compensation amount.

Why It Matters

Under- or over-funding employer contributions can trigger testing issues and require corrective adjustments with lost earnings.

Ways to Prevent It

• Review your plan document and payroll process regularly.
• Ensure your match formula is being calculated correctly.
• Address discrepancies early, not at year-end.
• Work with a 401(k) advisor to evaluate your options and strengthen your oversight process.

4. Using the Wrong Definition of Compensation

What Happens

The compensation used to calculate deferrals or matches doesn’t match the definition stated in the plan document (for example, bonuses or overtime are incorrectly included or excluded).

Why It Matters

This can lead to contribution errors that impact both employees and testing results. Fixing these issues retroactively can be time-consuming.

Ways to Prevent It

• Review your plan’s compensation definition annually.
• Coordinate with payroll to map pay codes correctly.
• Conduct a sample audit each quarter to verify calculations.
• Work with a 401(k) advisor to evaluate your options and strengthen your oversight process.

5. Failing to Update or Follow Plan Documents

What Happens

Plans must adopt mandatory amendments by IRS deadlines or risk disqualification. Sometimes plan terms aren’t followed accurately because internal procedures are outdated.

Why It Matters

Failure to follow or update your plan document can cause major compliance problems, even if day-to-day operations seem fine.

Ways to Prevent It

• Keep a plan amendment calendar for required IRS updates.
• Review your Summary Plan Description (SPD) regularly to ensure it matches current practices.
• Work with a 401(k) advisor to evaluate your options and strengthen your oversight process.

Bonus: Communicate Clearly with Employees

Many plan errors start with misunderstandings — either about eligibility, match formulas, or contribution timing. Regular communication helps prevent confusion before it becomes a complaint or correction.

Provide employees with:

• Enrollment reminders
• Contribution limit updates
• Clear explanations of employer match timing

Mistakes in 401(k) plans happen, but most can be avoided with careful oversight and regular check-ins between HR, payroll, and your plan providers.

• Review your plan operations at least once a year.
• Document processes and testing results.
• Partner with a qualified 401(k) advisor, TPA, and recordkeeper who can help you identify gaps before they become compliance issues.

Every plan is unique, and the right solution depends on your size, structure, and systems.

Disclaimer: This material is for general informational purposes only and is not intended to provide legal or tax advice.