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This in itself means nothing, especially if it is an isolated incident. It is like an old habit. However if other things along this line occur then there may be reason to be concerned. There is no reason that you should not bring this to his attention. If he is trying to hide something it is almost impossible to do with all the time and interactions that you have together. So based on this one incident, it does not sound as if there is reason to be concerned, but again if it this is part of a pattern, it may indicate that she is still on his mind. That does not mean he does not love you or loves her, but would be indicative that that the emotional attachment has not completely dissolved. So time and behavior will be the necessary to get the the answer to this question. Right now all you can act upon is speculation which I would advise you to avoid. I realize it is difficult, but try to be patient.I wish you the best and again advise patience.
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First I think a differentiation should be made between your mother in law and your son. I do not know here age but it is more difficult to remember things which is a natural occurrence associated with age. It is also not important that she accept the change of situation as you are not married to her. With respect to your husband that is a different issue and of more concern. If and when it happens again, ask him if knew that he called you by his ex's name. It is perfectly reasonable to share that it is disturbing and he need pay heed to that. It is also a good time to ask if it was just a slip or if she is still on his mind? By other things along this line I was referring to such things as mistaking your favorite food or wine, or the type of clothes you like. In other words things that could indicate he was confusing you with her/ I hope that this has clarified things and if not please feel free to ask.