While the preferred approach is to obviously transfer the assets back to the appropriate plan/trust with an interest/earnings adjustment as soon as possible (and should be done in any event), technically you may have a prohibited transaction (employer and/or fiduciaries utilizing plan A assets for plan B obligations). The employer may need to consider filing an IRS Form 5330 (Return of Excise Taxes Related to Employee Benefit Plans) and possibly a DOL Voluntary Fiduciary Correction Filing (benefit payments based on improper valuation of plan assets, depending on the underlying facts). Many employers/plan sponsors will likely choose to make the adjustment (no harm/no foul) without a filing and take the position that the IRS/DOL will not pursue if no loss in benefits/harm to participants or the ultimate amount of plan assets; others may treat as simply an "administrative error." Other considerations may include, e.g., possible violation of any reps and warranties in sale agreements or other financial commitments (if a prohibited transaction). Also, Form 5500 (under penalties of perjury) has a line asking whether there have been any prohibited transactions during the year; also, the independent auditors for the plan(s) may flag the corrective transfer in their opinion.
