The other kind of leakage happens when the capacitor physically leaks its liquid electrolyte. This can occur in a very dramatic way if an electrolytic capacitor is connected backwards, or if an internal short circuit develops due to a manufacturing defect. read more
I just found out that some capacitors hardly leak whereas other types of capacitors leak a lot of current through the dielectric. I’ve looked at Wikipedia and found several links (Leakage and Capacitor plague) which does not really described the current leakage (to the best of my understanding). read more