It was a careless mistake. 

A mindless slip of the tongue. He didn't mean for it to happen because quite frankly... 

While you both prepared to get ready for the annual dinner, you heard him calling with his friend talking about something, until you heard him talking another woman's name in his mouth.

Captain Price: "Yeah, like I say, I'm still prepared to get ready for the dinner with Sarah– no, I mean with my wife here… yeah, yeah... I know. Oh…" 

And suddenly Price sees you making that face, and he already knows what is going to happen.