Impeachment is the introduction of evidence (like documents or witness testimony) that contradicts a witness. When you are allowed to bring in impeachment evidence may be different in a proceeding other than a trial, but it is likely the same. In trial, a party is allowed to withhold the names of witnesses and documents whose sole purpose is to provide impeachment. The idea being that if the other side knew the document or person was going to be presented at trial, they would prepare by adjusting their testimony.
The problem many people get into in relying on the impeachment exception to including witnesses on their pre-trial witness list is that they misunderstand the difference between impeachment evidence and evidence that simply controverts the other side's position. You cannot hold off on identifying a witness who will come in and testify strongly on your behalf and claim that witness is impeachment. Impeachment does not relate to the other side's legal position. It relates solely to witness testimony. If the "impeachment witness" does not specifically impeach a witness, but instead provides evidence that hurts the other side, it is not impeachment.
Another problem that can occur is that the testimony may implicitly impeach a witness' testimony, but is not clearly contradicting the witness such that it will not qualify as impeachment.
A final problem is that a party will withhold the identity of a witness on the basis of the impeachment exception, and then they are precluded from using that witness to his or her fullest extent because the witness will be limited to impeachment testimony and nothing else.
Good luck to you.
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