When an argument implicitly assumes that a specific member (or subset of specific members) of a wider class is the wider class. Similar to Fallacy of Composition in transferring one thing's properties to everything else in its class, and overlaps at times with False Dichotomy (which occurs when two members of a wider class are claimed to be the only members of the wider class and that a choice must be made between them). This fallacy is often caused by an unstated premise.
"An Egoist theory of ethics is a contradiction in terms".
This assumes that "ethics" is a synonym for "non-self-interested."
"Anarchism is not a political ideology because politics is about the role of the State; advocacy of a stateless society is not a political position."
This assumes that the role of the State must be an active one—ie, the State must exist. (This applies whether one is arguing that Anarchism is not a valid political position, or that Anarchism is somehow "above" politics.)
Looks like this fallacy, but is not:
This should not be mistaken for an equivocation, which it can resemble. An equivocation hinges upon using the same word with different definitions.
"The judge has no interest in the case. A person who displays no interest in something probably won't pay attention. Therefore, the judge probably won't pay attention."
This is fallacious if in the first sentence, interest means "monetary stake or claim in the matter" and in the second means "the state of wanting to know about the matter."