I don't think you can any more, because the AIA changed that. (IANAL, get real legal advice if this matters to you.)

https://www.fredlaw.com/news__media/2014/01/13/521/aias_elim...

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In 2011, Congress passed sweeping patent law legislation, called the America Invents Act (AIA). The AIA changed many aspects of U.S. patent law, including the consequences of failing to disclose in your patent application the “best mode” you are aware of for practicing the invention. Traditionally, if you failed to disclose the best mode in your patent application, then that could be grounds for invalidating the resulting patent. The AIA changed this; no longer can patents be invalidated for failing to include the best mode. The AIA, however, did not change the basic legal requirement that inventors and patent applicants must disclose the best mode in their patent applications. In effect, the AIA left untouched the best mode requirement, but eliminated the punishment, or at least the main punishment, for failing to comply with the requirement.

if there's no punishment, then why would you expect them to comply?

Would this mean that, while you will not get punished for not disclosing the best mode, you can not call it a trade secret either (since you were expected to disclose it in the first place)?

I think it's kind of the point that they don't.

Technically, jaywalking is illegal...

And sometimes, jaywalkers get run over.

Or ticketed.

True story, I've shared here before: many years ago, a co-worker had to run home for some reason in the middle of the afternoon. On his way home, a woman jaywalked in front of his Escalade on a street with a posted 40mph limit. (He wasn't driving that fast, because he was still accelerating away from the stop light.) He hit her, watched her head bounce off the hood of his vehicle, and her go flying to land in a heap a few dozen feet in front of him.

He called 911, and, because there was a fire station like half a block away, the medics were on scene immediately, and the police shortly after. The policewoman who took his statement thanked him when she was done with that, and said, "Now, if you'll excuse me, sir, I'm going to go write her a ticket."

He responded, "Do you have to? She's already had a bad enough day."

She stopped, looked him in the eye for an uncomfortably long moment, and said, "Sir, you want me to write her this ticket..."