Yes, it happens.
In my case it was ME that started to say the wrong name, and it didn't happen waking up...it happened during a sexual encounter. I recognized it half a syllable in and tried to choke it back...and it ended up coming out like a name similar, but not identical to MrS's...I thought it had gone unnoticed but later Dude asked me "Who the hell is 'Bob', anyway?" We laughed a lot and told MrS about it later.
Dude now tells me that, during sex, I used to get it wrong quite a bit and never even noticed. He decided to just take it as a compliment, I was so caught up in my sensations that I was delirious with pleasure and just came out with my "default" name (After all Dude was my first male partner after being with MrS for 19 years).
I wouldn't read too much into it - just feel confident that he feels so comfortable with you that his auto-pilot morning routine kicks in. Next time (if there is one) just say, "Yes, I'd like some coffee..and I am Cee, by the way." Then make a joke about the fact that he is SO sexy that he has SO many lovers he can't even keep track of them ALL.
Me: poly bi female, in an "open-but-not-looking" V-plus with -
MrS: hetero polyflexible male, live-in husband (24+ yrs)
Dude: hetero poly male, live-in boyfriend (5+ yrs) and MrS's BFF
SLeW: platonic girlfriend and BFF
Lotus: "it's complicated"
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe