Some untrue assumptions people (including me) have learned from interacting with commercial sites and social media and taken to the fediverse:

The idea that if you post something that is copyrighted, you are the copyright violator. Loads of people post stuff on Twitter and Tumblr that they've copied from other sites and they just don't care, which is their prerogative. But on the fediverse each post is copied to and shown by many other servers run by other people. All these people are now also the copyright violator and can be contacted by lawyers from all kinds of companies.

(most platforms contain settings that the admin can set to make sure external posts are not shown to people who aren't logged in. I highly recommend making these settings)

(also note that Google doesn't know which posts belong to which server. Your own post will show up as belonging to some other server, and the server you run yourself will seem to host some very bad content. To prevent this, besides doing ample blocking, you should also make the same settings mentioned above)

The idea that the database administrator of the server you use, the one that can see everything, is somehow a trusted individual. In commercial social media and sites these people are screened by the city, the policy or even the security services depending on the kind of data involved. And these people are monitored by their companies. Here on the fediverse, anybody can be an admin. There's no control, so from the user there should be no trust.

(so don't post anything that you wouldn't want to become public, including when you are using those non-public posting options, including when saving a draft)

A minor thing: we assume that retweeting (sharing, boosting, repeating, whatever) is ok, because we do it all the time on Twitter. However, it is only ok there because the Twitter terms you accepted had a clause that allowed others to retweet (and embed) your posts. Most fediverse terms forget to include this, so in theory boosting etc is illegal. But I think we can all assume that the intent of every poster was to allow sharing, so this is only a minor misunderstanding.

(it is why I mention in my profile that my posts are cc-by, which essentially covers re-sharing using appropriate mechanisms)