In Washington State does the cop have to give you a ticket for what you were pulled over to ticket you at all?

Question

In Washington State does a cop have to ticket you or give you a warning for what he originally pulled you over for to give you a ticket or arrest you for something different?  What legally is a warning?  Can he just tell you that he is giving you a warning or does it have to be a written warning?

Antone Alfred Weber · Answer

So long as an officer has a reasonable expectation that a traffic violation or crime has been committed, the officer has a valid reason to stop a vehicle.  That the officer discovers a second violation, that they may have not had a way of knowing about until the vehicle as stopped, is also reasonable.  And example might be that the officer observed that the color-coded date sticker on your license plate was not up to date, but one the driver was contacted , they could not provide proof of vehicle insurance, or that they exhibited conditions of being under the influence of alcohol or drugs.  The officer does have the discretion of not citing you for the original reason for the stop, but you can bet that reason will be in the report materials to establish probable cause for the stop. 
 
Or the office might have discovered, once they got your paperwork, that you had not signed your registration (yes, you can be cited for not signing your registration.  It is a non-moving violation with a $139 fine).  But they may choose to verbally warn you of that violation.  No, they are not required to do so in writing.  But they have given you a break by doing so.

Q: In Washington State does the cop have to give you a ticket for what you were pulled over to ticket you at all?