I just made this mistake a few weeks back.

An interview was held by senior management to select a new supervisor for my department. I fell short of the other guy. A few days later, my manager approached me, expressed his dissatisfaction with the outcome and said he intended to promote me to a new position that he believes will benefit all. He just had to work out the kinks with my new supervisor.

One day, my new assistant supervisor and I were having a conversation, and I expressed my frustration to him regarding a significant issue that remained unsolved.

One week later, I received my first-ever verbal warning citing unprofessionalism, lack of social skills, my performance not aligning with departmental expectations given my many years of experience, not following the hierarchy etc.

They couldn't provide concrete examples of my negative critiques, but it does not matter. The email they wrote crucified me.

I have been waiting to hear back from my manager since that meeting. It's not going to happen.

I should have kept my damn mouth shut.