If you are cloaking and have a halo equipped, the halo will give away your position!
I learned that the hard way after getting dominated constantly and wondering why.
This is why I play the New unreleased class that Valve has let me beta test.
It's a mix between spy and demolitions man, In a nutshell, it has sticky bombs, with all the bells and whistles of a spy.
<3 Unreleased Beta classes


Are they really making a new class?
Holy spies cannot be backstabbing criminals.