Washington is an at-will employment state, and as such employees can be subjected to adverse employment actions (termination or discipline) for any reason or no reason at all. You can actually be terminated from employment even over a mistaken belief that you did something wrong, provided the dismissal is not discriminatory. Once an employee reports co-worker harrassment to an employer, the employer is then on "notice" that the incident occurred and likely the employer was trying to protect himself by issuing a reprimand to you. Were they to ignore the problem it could create legal liability for them down the road. My suggestion is to keep it professional with the reporting employee and only talk about work. That way the chances of this happening again are reduced.
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I agree with the other attorney that has answered this question. I only add more here because you should really consider whether you would tell a judge at a traffic hearing, "but everyone else was speeding, too! How can I get a ticket when none of them were pulled over?" That defense doesn't fly except for two situations: (a) if you are a public employee, you are entitled to the same discipline that others have received in the same circumstances, if your prior disciplinary records is also the same; or (b) if you have reason to believe that you are being targeted for different treatment because of your own legally protected status (such as your race, disability, prior whistle-blowing, etc.) Such circumstances are VERY rare, so I tend to believe that the first answer you received probably applies. If you still have doubts, consult counsel.
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