Brent Ozar takes us through a couple considerations when using online, resumable index creation:
In SSMS, you’re used to being able to click the “Cancel” button on your query, and having your work rolled back.
You’re also used to being able to kill a query, and have it automatically roll back.
Neither of those are true with resumable index creations. In both cases, whether you kill the index creation statement or just hit the Cancel button in SSMS to abort your request, your index creation statement is simply paused until you’re ready to come back to it. (Or, it’s ready to come back to haunt you, as we saw above.)
There are some good things to keep in mind here.