It is as unacceptable to harass a user with a history of inept or disruptive behavior as it is to harass any other user.Wikipedia encourages a civil community: people make mistakes, but they are encouraged to learn from them and change their ways.
The important component of hounding is disruption to another user's own enjoyment of editing, or to the project generally, for no overriding reason.
If "following another user around" is accompanied by tendentiousness, personal attacks, or other disruptive behavior, it may become a very serious matter and could result in blocks and other editing restrictions.
Correct use of an editor's history includes (but is not limited to) fixing unambiguous errors or violations of Wikipedia policy, or correcting related problems on multiple articles.
In fact, such practices are recommended both for Recent changes patrol and Wiki Project Spam.
Harassment is a pattern of repeated offensive behavior that appears to a reasonable observer to intentionally target a specific person or persons.
Usually (but not always) the purpose is to make the target feel threatened or intimidated, and the outcome may be to make editing Wikipedia unpleasant for the target, to undermine, frighten, or discourage them from editing.
A pet rat might intimidate your sister's friends, keeping them out of your fort.
"To frighten" or "make fearful" is at the root of the verb intimidate.
These words appear in red, and are graded with stars.
One-star words are frequent, two-star words are more frequent, and three-star words are the most frequent.
Many users track other users' edits, although usually for collegial or administrative purposes.