1. Accessing Non-Public Areas
While the first place that you look might be Twitter's Terms of Service, most of these are things that the average person isn't likely to do on accident. These include things like deliberately hacking Twitter, or "accessing, tampering with, or using non-public areas of the Services."
In short terms, "no accessing Twitter through backdoor channels meant for developers." While it is possible to find a backdoor on accident, Twitter asks that in this event you report the issue to them so that they can fix it.
2. Doxxing
The above items involve using Twitter features and tools that you haven't been given access to. However, Twitter also forbids using public features and tools in malicious ways.
In this category, Twitter's Terms of Service specifically mention using the service to do things like spread viruses. It also includes "doxxing," or, sharing personal identifying or location information of other people without their permission or consent.
3. Posting Intimate Media
Twitter's documents recognize a difference between "private" content and "sensitive" content. Twitter defines "sensitive media" as violent or adult content.
There are some places where this content cannot be shared at all, and some content may be so sensitive that it is not allowed anywhere on Twitter. However, accounts have more leniency in these matters for artistic or documentary purposes if they manually mark their accounts as sensitive.
4. Impersonation and Defamation
The Terms of Service also explicitly forbid sending messages that appear to be from Twitter. So, taking the Twitter logo and using it in your own emails is violating its terms and conditions.
5. Zero-Tolerance Violations
Twitter includes "hateful" content in its list of sensitive media, but "Hateful conduct" is also its own category of content that promotes violence against a person or group of people. "Violent threats" are one of only two actions that result in "immediate and permanent suspension" of an account.
