John, one of the the worst things about overfilling apart from leaks is it can also cause aeration problems,(the foamy fluid Joe H mentioned)this is due to the fluid being churned up by the drums and planetary gears.
Aerated fluid takes longer to pressurize the hydraulic circuits.

It can also cause problems with the valve body,causing slippage and consequently making the trans run hotter,which leads to sludge and varnish problems.

Remember: There is only 1 pint difference between the add and full marks,it should be between the two at running temp.

You get the picture......

Last edited by jockbolter50; 09/12/2011 8:29 PM.

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.