Sloped drop shadow
How to use it
Add a sloped shadow with a blur between two elements where one is closer to the viewer than the other.
When to use it
What to watch out for
In the example below I gave the sloped shadow a slightly rounded top, and a blur. This gives it a softer, more natural feel.
This sort of sloped shadow might give the impression that the light source is hitting the elements from a strange angle, or that one element is leaning against the other.