While I don't practice in Washington, usually this is the kind of deficiency that would lead to a dismissal. A ticket or summons must place the defendant on notice of the charge, so if the ticket is in error or charges you with doing 25 mph and thats the speed limit. then in NY (where I practice ) the case would be dismissed. I would call a local traffic attorney in your area for a consultation.
It's a good idea to fight your ticket if you want to save money on your insurance premiums and to keep your driving record clean.
Certain errors on a ticket, but not all errors, can lead to a dismissal. But different judges have different views on particular errors.
I'm a little confused by your posting. You say the officer wrote your speed as 25 mph when he should have posted it as 40 mph. Is that correct? Then you said when you got home, the speed limit sign said 25 mph. I just wanted to make sure I understood your posting.
Regardless, this seems like a clear case of where the ticket would be dismissed outright. It is about the same as the officer writting down the wrong statute that you violated. Even if you appear and plead no contest, that should keep you from having points assessed against your license.
If I were you, I would contact the clerk and request a hearing. If you are not familiar with this process, it is fairly simple. A date will be set where you typically go before the judge in a room with about 20 or more people that are contesting their traffic violations. It's then up to you whether to plead not guilty or no contest.