"That's icing (or frosting) on the cake!"
This can phrase can be used in two ways;
1. An additional good thing to a circumstance that is already good.
----Ex. "I passed my big test! And I finally got that bonus! Talk about icing on the cake!
2. Alternatively, you can use it to mean something else bad happens on top of an already bad circumstance. Generally, this is used very sarcastically.
----Ex. Your car breaks down on the highway as you were trying to make your way home from work and when you go to reach for your phone to call for a tow truck, you find that you forgot it at work.