Heat can wreak havoc on the plastics in certain weighted keyboard action designs. The weights will expand at a different rate than the plastic used to capture the weight and eventually that plastic will crack, resulting in a resounding "clack" when the key is pressed.

Heat like you mention here can also be detrimental to certain types of LCD displays, as well. Sometimes the display will darken completely but will come back when the temperature falls to normal room temp, but sometimes the damage is more permanent, resulting in anything from a dark blotch where nothing can be read to the fullscreen becoming useless.

Rule of thumb for ANY musical instrument is to not leave it where a human wouldn't be able to survive for long.