I have a 2006 Chevy Colorado and have noticed some little puddles of transmission fluid under it. With the engine cool (overnight), I cleaned the dipstick, put it back in and then checked it. The fluid was way way above the cool level and into the hot area. I haven’t ever added fluid so maybe the guy I bought the truck from a few months ago put too much in? Would that be causing the leak?
I don’t know about your car, but on many you are supposed to check it with the engine hot and idling in park (on the level). You should check–possibly the owner’s manual. It could be far above the proper level, and this is indeed a BAD THING if true.
+1 for @melott. Your truck must be running and warmed up on level ground when you check it. Automatic transmissions are hydraulicley operated devices and when you shut them down most of the fluid will drain back into the pan giving you a very high reading. If it turns out you do have a leak check the transmission cooling lines first.
When overfilled, fluid can be pushed out the case vent. Erratic shifting would likely occur when overfilled to that extreme though.
Overfilling will cause the transmission to overheat enough to cause significant damage. It’s better to be a quart low rather than a quart over full.
Plus 2 to melott. It’ll show over full until the fluid is circulated and then heated to get the right reading. But they may say something different in the manual. I suspect you have a pan gasket leak or the bolts need to be torqued a little. A good time to just have it serviced anyway with a pan drop and new filter. Its just good maintenance every 30K.