normally, when you overfill anything, be it engine, transmission, or diff. it will cause the fluid to churn a lot, and cause aeration and bubbling. if serious enough, it could lower the viscosity of the fluid, and cause overheating and premature wear. how exactly are you checking your fluid? the correct way to check tranny fluid is: drive till vehicle is at normal operating temp (this could take 15 miles or so for a tranny to fully heat up), park on a level surface, apply parking brake, put trans into neutral, and check the fluid.