Test-test, where a test contains multiple subtests that need to execute.
People in a table for genealogy, through a join table that denotes children.
Servers where one server is a master and the other(s) replicate.
And, since this question is remarkably open-ended, might as well give some docs on parent-child dimensions.
Looking at stack overflow a bit more, it appears this is a dupe of here, though: When we need to use 1-to-1 relationship in database design?