A380-800 driver, easyduzzit;
Your above statement is incorrect. You won't get any warnings if you stuck a higher flex temp in when the actual performance require a lower Flex temp (or none at all).
No, I know you won't get a warning if you stick a higher temperature than the proper flex temp - my comment was in reference to the position of the thrust levers - if they're not in either the TOGA or FLX/MCT detent, the warning I described will sound. Thanks to both of you for sorting out the higher flex temp item; yes, the engines will run "at the flex temp entered".