I watched Phantom Menace for the second time last night (yes, it took me that long to venture into Jar Jar territory again) and noticed that Queen Amidala was referred to as both ‘Your Highness’ and ‘Your Majesty’ - sometimes in the very same scene.
Was this a mistake, or is it possible to refer to a Queen in two different ways? I always thought the former was reserved for princesses and princes.
She’s an elected Queen. I don’t think it matters.
I think “Your Highness” is the preferred term.
Actually that’s just because I like how Han Solo says “Your Highnessness”.
I’ve always been partial to “Your nipples.”
If you want to nitpick, and use real-world twentieth-century usage, a queen should be “Her Majesty”.
Depending on the type of principality, a princess could be:
Her Imperial Highness
Her Royal Highness
Her Highness
Her Serene Highness
Her Excellency
I have no idea which would apply to Princess Leia. Then again, Luke once addressed Emperor Palpatine as “Your Highness”, when the correct form would have been “Your Imperial Majesty”. I don’t think George Lucas is that fussy about protocol.
Queen Elizabeth is properly addressed in conversation as “Your Majesty” but can also be addressed in the third person as “Her Royal Highness” - you’ll see “HRH” often slapped in front of her name.
Princes and princesses are “Your Highness.”
It’s all very strange. Since she is the Queen of Naboo: A WHOLE PLANET, I would say that qualifies her for Imperial Majesty, but a) it’s an elected post (whatever), and b) the Gungans don’t recognize the Naboo as their government.
In other words, it’s long ago in a galaxy far far away, so it doesn’t have to make sense. Either one is correct, since they say both.
A followup question. What is Leia Princess of? Since Amidala’s title was by election, she could not have inherited it. Was it only a courtesy title? Anyone know?
Me either, but I do think we can rule “Her Serene Highness” right out. 
Wasn’t Leia’s title from her adopted family, anyway? So she’d be a princess of Aldaraan.
That, or she’s just a princess of “damn, she’s royally hot”.
For Majesty, yes, but not Imperial Majesty. Imperial Majesty implies that the monarch is an Empress or Emperor, which would mean that she is sovereign over some other monarch or another sovereign or semi-sovereign nation. Amidala was queen only of Naboo, with no subordinate royal class, and so could not claim the title of Empress.