The Martens group has recently proposed a method to model quantum transitions using classical-like trajectories.  In order to capture the quantum interferences in this fully coherent approach, the usual definition of probability must be generalized to allow quantities that can be greater than 100%--or even negative. 

Here is the link to the paper:  http://scitation.aip.org/content/aip/journal/jcp/143/14/10.1063/1.4933049