Short story: what the title says.
This is especially problematic for folks who merge multiple databases onto the same server. All it takes is one badly-behaving application to leave its transactions open, and suddenly it causes the rest of the databases to run TempDB out of space. That app’s transaction might not even be changing anything, and it might have never caused problems for that application before – but once it shares a TempDB with other apps, it causes cascading problems.
Long story: let’s start by creating a database and throwing some data in it:
CREATE DATABASE ShouldBeIsolated; GO ALTER DATABASE