Right. If one expert can make a compelling case to a jury, that jury will set a precedent for future defense lawyers. In fact, I'd be surprised if it isn't already the case in some jurisdictions.
The precedent affects the directions given to the jury by the judge, and can generally only be changed by an appellate court ("the direction given by the judge in x case was incorrect and therefore the conviction should be overturned/the case retried").
Regardless of the direction given, juries are generally entitled to return whatever verdict they want (see jury nullification). If a judge (correctly) directs them that a sniffer dog alert constitutes cause for a search they may still acquit.
I think you're confusing legal "precedent" with the colloquial meaning of the word.
Isn't it obvious that I meant "precedent" in the sense of "a jury found this expert's testimony to be credible once, and therefore is likely to do so again"?
Seems like the "for future defense lawyers" should have been enough of a clue to head off the pedantic hair-splitting.
