Wikis are community-based projects, where people from all across the world can come together and share their passion for a subject to a collaborative end. It goes without saying, then, that you may not agree with everything someone does. The important thing to remember is to assume someone is acting in good faith. Most people are on a wiki to help it, not to hurt it!
What is good faith?
To assume good faith means to assume that someone is making a well-intentioned effort to help the wiki, even if they did something you think is wrong. Odds are, it was an accident. Assuming good faith means to assume that there is no intention of malice, that editors are trying hard to do their best, and that they are trying to do their best for the greater good of the community.
Even if someone made an edit that needs to be reverted, it doesn't mean that their intention in making that edit was anything less than honorable. You should approach disagreements with a sense that the other person just wants to help, so you can be a friendly, honest, and caring voice in the community rather than someone who assumes that anyone who does something differently than you is out to ruin the wiki.
Assuming good faith from new users
Making the leap from reader to editor can be really daunting, and the Publish button can be very intimidating when you know your first edit can be seen by the entire world. For that reason, you should always remember to be patient with newcomers, because they will most likely be unfamiliar with the wiki’s culture and rules. They are there because they are excited to join in, but they may not understand the tools or codes that you're used to, and they most likely have not read your wiki's rules and guidelines—no matter how much you may have wanted them to. It's easy to get frustrated and impatient about this, but everyone needs a chance to learn—and at a pace that's as comfortable for them as possible.
When you can reasonably assume that something is a well-meaning error, correct it with a kind, explanatory edit summary, and even leave a message on their user talk page or wall. Don't just revert it without any explanation, and certainly don't label it as vandalism unless it actually has malicious intent behind it. Letting the users know what they did wrong not only helps them become a better editor, but the new messages bubble showing them that someone has in fact read their edits can be great for positive encouragement. Knowing that someone has read what you wrote is a great feeling, one that can get people deeply involved in wikis they care about.
All in all, remember not to act as if their mistake was deliberate. Correct them, but don’t scold them. Inform, but don’t intimidate. These are people you want to keep on your wiki, so why scare them off?
Different people can have different opinions about what's best for a wiki. When you disagree with someone, you might start to feel that they are a bad editor, but that's almost never true if they care about the topic. Assuming good faith is all about intentions, not actions. Even if the person is wrong, that doesn’t mean they’re trying to ruin anything.
Whenever an edit or a message irritates you, take a step back and assume the person is just trying to help. This can help you look past your frustration and recognize what they are trying to achieve, and then act more kindly based on that understanding. Consider using talk pages, comments, and/or walls to explain your point of view, and invite others to do the same. This can avoid misunderstandings and prevent problems from escalating.
Well-meaning people can take actions the rest of the community feels are unwise, and you should discuss the actions calmly and seek compromises wherever possible. What you should not do is accuse the person of vandalism or sabotage. Foster a culture of consensus, not condemnation.
A note for admins
When you talk with editors, you should always be friendly and patient. Show them that you are assuming good faith about their intentions, whether they are new or regular editors. This prevents or at least calms most conflicts, and it helps the community trust you. It also sets an important example for other editors and admins, which can lead to a more welcoming and positive atmosphere on the wiki.
When a well-meaning person takes an action you disagree with, you should always start with a message on their user talk page or wall. Never go straight to blocking them or deleting or protecting a page—and never use admin tools or stature just to get your way in a disagreement. Remember that every action on a wiki can be undone, and letting an unwanted edit or page sit for a short time while you talk with the user and wait for their reply is not a terrible thing.
Of course, there are users who act in bad faith, and we trust admins to use their judgment and their tools to deal with them properly. However, assuming good faith is a powerful tool. Even in cases of obvious vandalism, you might be surprised how often a kind and personal warning can get people to stop, and even apologize. Some vandals are just bored readers who don't think anyone is actually paying attention, or don't think that their obnoxiousness is affecting real people.
Really malicious cases of spam, vandalism, and trolling are generally blatantly obvious, and those can be dealt with quick administrative action like blocking. The vast majority of people are there to help the wiki, but every once in a while you get someone who just wants to vandalize to ruin other people's good time.
Just remember that everyone you deal with has feelings, and everyone can help you build the wiki project that you love if they can be shown that their work is noticed and valued, as well as how they can improve wherever needed. Always do your best to assume good faith, and your reward will be a thriving wiki community!