Alex has ultimate authority on the site. It is he who appoints and dismisses moderators. Sometimes mods are fired, occasionally they may receive a rebuke. Sometime we rebuke ourselves. Yesterday I said something rude to leejosepho in the heat of the moment as he was driving me insane with his comments in a PPR thread, but I apologised straight away - we are human, we do make mistakes and we do get annoyed occasionally. However, I feel we do a good job overall.
If anyone feels a moderator has acted unjustly, report it to Alex. Your complaint may be valid and upheld or he may reject it. Typically if the other mods see a pattern of inappropriate behaviour by another mod, we try to guide that mod in the right direction, failing that we may ask Alex ourselves for that moderator to be removed.
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I've left WP indefinitely.