You would report it in Other Reportable income as a negative number for a loss.
In TurboTax, open your return and follow these steps:
- Click on Federal in the left-hand column, then on Wages and Income on top of the screen
- Scroll down to All Income, locate the Less Common Income section
- Click Show more and click Start next to Miscellaneous Income at the bottom
- On the next page, click Start next to Other Reportable Income.
- Enter your loss as a negative number and describe it as Loss on excess contribution to 401(k)
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