When you over fill a gear case, diff, crankcase etc., the fluid has no place to expand as it heats up and will be forced out the breather. It can cause seals to leak if there is not enough relief being provided by the breather also. The other problem that can occur is the fluid gets too much mixing action and becomes thinned out with air and loses it lubricating abilities, this is why it is a very bad idea to overfill a crankcase. In your case you are probably going to pump out some fluid and make a stinky, sticky mess.