Excerpt: [A Solo 401(k) plan] must meet certain ERISA and Internal Revenue Code requirements. And one of those requirements is the obligation to file Form 5500-EZ if plan assets exceed $250,000. And here's where there could be bad news. But sometimes that requirement gets lost in translation, and a self-employed or small business owner whose plan exceeds that threshold doesn't file the return. It may be because he or she missed the filing after being exempt for several years before the $250,000 threshold was crossed. Or it may be that the financial services firms at which these plans were established did not inform the self-employed or business owner of the filing obligation.