Well a 'standard rule' is is not a law of nature - it is the advice that will, more often than not, be correct but it doesn't mean there aren't exceptions.  But be careful - companies especially will often push their luck on these kinds of things.
A few years ago a famous AI researcher resigned from his position at Sega (where he was researching better AI for them to use in their video games) to take a job with the American company ID (both were, at that time, major players in the industry and prime competitors).

Sega then proceeded to sue him, and ID, claiming that whatever knowledge he has gained in his field of expertise (AI) in the 15 years he worked for them were covered by the NDAs he had signed and basically he wasn't allowed to ever work in that field again ! 

They would probably have lost the case since no American judge would accept that an NDA can bar you entirely from a career path. But it never went to trial, ID settled the case.

And Sega got a nice payout for nothing.