What can damage your automatic transmission

Most of the transmission troubles start from overheating.
Under heavy load, such as towing a heavy trailer, rocking the vehicle from the snow, having continuous stop and go traffic in hot weather, racing, etc. the transmission overheats. At higher temperatures the transmission fluid burns, losing its lubricating qualities and becomes oxidized leaving deposits all over inside the transmission. Exposed to the heat the rubber seals and gaskets inside the transmission become hardened causing leaks. The metal parts warp and lose their strength. All this, sooner or later, results in transmission failure. For example, a friend of mine burned the transmission when he was spinning the wheels too hard trying to free his shiny Audi from the snow on the next day after he bought it!
However, overheating is not the only reason - sometimes transmission breaks down because of poor design, due to lack of maintenance or after being rebuilt by inexperienced technician. Few other reasons: harsh driving, too low or too high transmission fluid level or wrong transmission fluid type - a person I know added gear oil into the automatic transmission... guess what happen, the transmission died after 40 minutes of driving!