The worst cases are easily those who not only are very ban-happy, but who ban out of the blue.
I've seen grids which included into their rules that they reserve themselves the right to ban anyone for any reason. Like, on a whim. Upon first strike. With no explanation whatsoever. Just like so. Certainly safe environments, especially for those who aren't BFFs with the whole grid staff.
"You did something in our grid that we don't expect anyone to do: BAN!" (This could probably something as basic as walking along an existing path if the grid owners expect everyone to use the teleporters for convenience.)
"You're friends with/you don't take position against X: BAN!" (Bonus points if they come and seek an alliance with you against another "foe" a few months later.)
"We know from hearsay that you were at Y's party six weeks ago: BAN!"
"When we started this new weekly event, you refused to stop your own simultaneous one which we see as competition: YOUR WHOLE HOME GRID IS BANNED AND SO ARE ALL GRIDS THAT ARE ON YOUR SIDE!" (And then they still expect to have at least 40 party attendees every week.)
"You've criticised/not agreed with me/my friend Z seven years ago: BAN!" (Again, until they need an ally.)
"You've reported a bug on one of my sims to me? BAN FOR PERSONAL INSULT! MY SIMS ARE ALL FUCKING PERFECT, BITCH!"
"You have an avatar on Y's grid? WE'LL BAN ALL YOUR AVATARS REGARDLESS OF GRID! AND WE'LL BAN ANYONE WHOM WE CONSIDER AN ALT OF YOU! FUCK FALSE POSITIVES, THERE ARE NO FALSE POSITIVES!"
"You use content from X on your avatar/on your sim? BAN!"
"Someone on my side has trolled/attacked/griefed you on OSW/in-world, and you've dared to defend yourself or even retaliate: BAN FOR YOU AND EVERYONE WE THINK IS YOUR FRIEND!"
In fact, why stop at banning if they can also block you on OSW and only unblock you for a minute or two to attack and harass you all the way to suggesting suicide or all-out death threats whenever the hell you post anything anywhere?
If you think this is bad on English-speaking grids, look at the German OSW scene. Have you ever come across American OpenSim users who, in all sincerity, not only want to see a certain US-based grid destroyed permanently, but also the whole grid staff physically dead in real life ASAP?
Well, then there are those who do go by defined rules, but who don't make them public. It's often a case of being too lazy to publish rules because they're allegedly common knowledge everywhere.
For example, such people may define the Adult rating as "G-rated, but no child avatars allowed". But they don't say so anywhere. Then someone who lives by Second Life's rules comes in naked. And they're banned on the spot. Oftentimes, the reasoning is that "no child avatars allowed" allegedly IS OpenSim's entire official definition of the Adult rating, and that allegedly everyone knows this anyway, so there's no need to write it down anywhere. That is, this is much more common for specific sims. But the only OpenSim grid that has ever published official written definitions of the ratings is DigiWorldz, and even they allow their sim owners to bend the Adult rating to "G-rated, but no child avatars allowed".
Speaking of child avatars, if they aren't allowed somewhere, there should be a definition of what's considered a child avatar. And that definition should be part of the rule, it should be written down somewhere where people will actually read it, and this definition and only this definition should be used to judge whether an avatar counts as underage.
Some sim owners or grid owners define "underage" by avatar height. They may even have a child gate in place. But they don't communicate anywhere what's the minimum height for an avatar to count as grown-up. And then they configure it to 6' or 180cm. Again, they don't say so anywhere because they think "everybody knows" that avatars below that height are kids.
Others say they decide on the spot whether an avatar looks underage or not. They don't have any fixed criteria, and even if they did, they couldn't be bothered to write them down. If your avatar feels underage to them, you're out. This, however, is a total minefield for all avatars that don't go the maxxed-out badassitude path for male avatars or the maxxed-out sexiness path for female avatars. See here:
https://opensimworld.com/post/108261
NB: I'm not an illegalist radical anarchist who breaks rules just because they exist and in order to try and make them disappear. I do try to adhere to rules whenever and wherever possible. In fact, Juno still has a bad feeling about visiting OSgrid Plazas wearing sheer tights along with underwear under a calf-length skirt. There's the "no exposed underwear" rule, other avatars' experience has proven that what's visible when camming up a skirt may pretty well count as "exposed", and sheer tights don't completely cover up the underwear underneath. The OSgrid Plazas are a case in which the rules are communicated, but where it isn't entirely clear to which extent they're applied and enforced.