Well, there are a few popular strategies here, and considerable debate as to which is the "best"... I'll lay them out and leave the decision up to you.
This strategy involves carefully monitoring a moderator's activity over time, potentially many months or even years. Notes are taken of any inconsistencies between what the moderator says and does, supporting information (transcripts, screenshots, archive.org links) are attached and annotated.
Finally, the archive is presented, either publicly or to the site administrators, along with the assertion that - in light of such overwhelming evidence of malfeasance - the only sane option is to remove the moderator from his position and send him away in disgrace.
In this strategy, one distills each utterance of the duplicitous moderator into its most literal form, bereft of all nuance, and tirelessly applies it in every possible context until the rest of a community is in an uproar over the resulting chaos... At this point, the robot presents his defense: "this moderator commanded me to do so. Your grievance is not with me, but with him!"
The Rage Quitter
Generally the easiest but least effective strategy, a rage-quitter responds to any official rebuke with a tirade of vulgarity and accusation, then leaves the site vowing never to return. Unlike the previous two strategies, rage-quitting has little chance of actually convincing anyone else that the dastardly moderator is actually in the wrong - its success hinges on the Quitter's ability to vaporize all semblance of order and dignity with his outburst, leaving behind a smoking wasteland for which the moderator must take responsibility. Once the dust has settled, the Quitter may quietly return to the site, disclaiming all knowledge of the distant past in which such alleged outbursts might have been said to occur.
This clever strategist does not seek to attack the wretched moderator directly, but instead calls him out obliquely - making references only to "certain confused users" who are "struggling to understand our ambiguous rules". The Legislator then draws up plans to "clarify" the rules, carefully defining each scenario in which they apply and the resulting implications in the most minute detail. The goal here is to prevent the moderator from speaking up without incriminating himself, while simultaneously tying his hands to prevent such meddling in the future.
Easily the most boring strategy of the lot, this strategist calmly talks to the moderator, perhaps even in public, laying out the behavior he found worrisome and asking politely for clarification or correction. Success in this strategy is anticlimactic; all but the most stupid moderators will recognize that the game is up and either explain or reverse their actions to avoid the wrath of the community. I honestly cannot recommend using this strategy unless you for some reason wish for a long and productive future on the site.