Absolutely real. I once had to replace the diaphragm on one of my previous horn tweeters; well the fixed channel was harsh and piercing for a couple days, skewing the stereo image to that side, until it settled down and both channels started playing equal.
Same with my new tweeters, I had to change the crossover settings after a few days because they started really hot and bright fresh out of the box and ended up smooth to the point of dullness until I changed the settings again.