Wow, saying it was an "oversight" or that they just didn't bother changing the script really sounds lame, and honestly, it has never even occurred to me that that might be the case. Even if the character was originally going to be male, no one noticed that they were calling an obviously female officer "Mister"? When they reviewed the dailies, no one said "What the hell? Why are they calling her 'Mister'?"
It had to have been intentional. She's Mr. Saavik enough times to make the point, Lt. Saavik most of the time, or just Saavik to Spock. There's no way that they oops forgot to change the script.