California Code, Code of Civil Procedure - CCP § 1258.290

(a) The court may, upon such terms as may be just (including but not limited to continuing the trial for a reasonable period of time and awarding costs and litigation expenses), permit a party to call a witness, or permit a witness called by a party to testify to an opinion or data on direct examination, during the party's case in chief where such witness, opinion, or data is required to be, but is not, included in such party's list of expert witnesses or statements of valuation data if the court finds that such party has made a good faith effort to comply with Sections 1258.210 to 1258.260, inclusive, that he has complied with Section 1258.270, and that by the date of exchange he:

(b) In making a determination under this section, the court shall take into account the extent to which the opposing party has relied upon the list of expert witnesses and statements of valuation data and will be prejudiced if the witness is called or the testimony concerning such opinion or data is given.