I made a 2 tiered cake filled with chocolate mousse, as I was finishing decorating it... I saw that some filling was coming out of it. When we got to cutting the cake the top tier just slided of! Could it be that I used to much filling? I just feel so bad about it.

Did you make your dam thick enough?

Did you use an icing "dam" around each tier before you added the mousse? How thickly did you apply your filling?

Always use an icing dam (1/2 inch or so thick piping of your buttercream around the outer rim of your tier...then fill with mousse) and thicker is not better. I apply my fillings only 1/4 to 3/8 " thick; thicker and the filling is squeezed out by the weight of the top tier(s).

Hope this helps.

Agreed, use a thickened buttercream for the dam to keep the filling in, but IN ADDITION, you cannot fill the filling up to the top of the dam or you will have a sliding cake. I'm guessing that's what happened.

Collapse is typically from inadequate or no supports.
Slide is typically from too much filling, however, cakes can also slide without any filling if a center dowel isn't used and the cake is moved.

Thank you all for your answers, actually I used a dam but I definitely went overboard with the filling it was as thick as half a layer, next time I won't go over 1/4, or over my dam. It was really a looot of mousse in the middle but I definitely learned my lesson. Thanks again!