29 CFR 18.103 - Rulings on evidence.
(a) Effect of erroneous ruling. Error may not be predicated upon a ruling which admits or excludes evidence unless a substantial right of the party is affected, and
(1) Objection. In case the ruling is one admitting evidence, a timely objection or motion to strike appears of record, stating the specific ground of objection, if the specific ground was not apparent from the context; or
(2) Offer of proof. In case the ruling is one excluding evidence, the substance of the evidence was made known to the judge by offer or was apparent from the context within which questions were asked. A substantial right of the party is affected unless it is more probably true than not true that the error did not materially contribute to the decision or order of the judge. Properly objected to evidence admitted in error does not affect a substantial right if explicitly not relied upon by the judge in support of the decision or order.
(b) Record of offer and ruling. The judge may add any other or further statement which shows the character of the evidence, the form in which it was offered, the objection made, and the ruling thereon. The judge may direct the making of an offer in question and answer form.
(c) Plain error. Nothing in this rule precludes taking notice of plain errors affecting substantial rights although they were not brought to the attention of the judge.