See the document section for a nice memo on the issue, but also see the following:

A few cases have approved the imposition of evidence and issue sanctions without a court order violation, but those cases involved egregious discovery abuses going far beyond the failure to supplement or amend a response in a timely fashion. (See Vallbona v. Springer (1996) 43 Cal.App.4th 1525, 1545 [evidence and issue sanctions improperly imposed without violation of order compelling production of documents, where requiring requesting party to seek such an order would have been futile in light of responding party’s claim that requested documents had
been stolen]; Do It Urself Moving & Storage, Inc. v. Brown, Leifer, Slatkin & Berns (1992) 7 Cal.App.4th 27, 36 (Do It Urself) [evidence sanctions appropriate despite absence of order compelling discovery, where sanctioned party concededly could not provide audit it had promised].)

See also
Thoren v. Johnston & Washer (1972) 29
Cal.App.3d 270
that “[t]he power of the trial court to bar the testimony of a witness willfully excluded from an answer to an interrogatory seeking the names of witnesses to an occurrence is found in the express language of the discovery act and is an inherently necessary one if the purposes of the act are to be achieved.”
(Thoren, supra, 29 Cal.App.3d at p. 273.) The court went on to opine that “[a] willfully false answer to an interrogatory must be treated as the equivalent of no answer at all . . . ,” and that “[w]here . . . that falsity lies in the deliberate omission of the name of a witness to the occurrence, an order barring the testimony of the witness must be sustained as a sanction . . . which the trial court properly ‘deemed just.’ ” (Id. at p. 274; see also id.
at p. 275 [because plaintiff “gave the name of only one person” in interrogatory answer, “the trial court could properly hold that he should be limited to calling that person to testify . . . .”].)

Often, even if you fail to disclose something you should have, if you are tricky and disclose it later in the case, and before trial, the judge will not penalize you. So if that happens, argue why the "evidence sanction" is warranted, or not warranted, again by looking to the Thoren court which said:

In Thoren, it was not until the start of trial, over two years after plaintiff served his interrogatory answer, that the defendant learned a witness had been omitted.
The court noted that because “the falsity of the answer was not discovered until a jury had been impaneled, [the] situation militat[ed] against solution of the problem by a
continuance.” (Id. at p. 275.)

In Thoren, before ruling on the motion to exclude the challenged testimony, the trial court in Thoren held a hearing, and concluded, based on substantial evidence, that at the time the interrogatory was answered, the plaintiff’s counsel either had actual knowledge of the witness’s role in the case, or deliberately refrained from
finding it out before answering. Thus, the court determined that the interrogatory answer omitting the witness’s name was not merely incomplete, but “willfully false.” (Thoren, supra, 29 Cal.App.3d at pp. 273, 275-276.)

Again, you still do not need to update your discovery responses. Also see Rangel v. Graybar Electric Co. (1977) 70 Cal.App.3d 943 (Rangel), the very same panel that wrote Thoren distinguished its earlier holding on precisely the basis that there was no showing that the responding
party had willfully concealed a witness’s name. The court cautioned in Rangel that “[i]n the absence of stronger evidence of wilful omission, to uphold the trial court’s action barring plaintiff’s rebuttal witness would permit the use of interrogatories as a trap, pinning a party for all time to an answer intended to reflect only that party’s knowledge as of the date of answer. [Citation.]” (Id. at p. 949.)