What is the difference between a warranted and an unwarranted assumption?

There is at least some reasonable and verifiable evidence behind a warranted assumption. I think that even with evidence, the consequences of an assumption have to be considered. Evidence is not necessarily proof. If harm will come to a person because of an assumption, then I would consider the assumption unwarranted. You can't go off hurting people because you think you're clever. That kind of behavior will come back to bite you.