It's not a one off, it's literally a good couple of times an hour.
DH noticed it first. Things like 'aww show mummy, mummy kiss it better' or 'go and see mummy' when he passed DS to her etc.
He does tend to act like I'm not DS's mum and they have a big deal about doing things 'their way' with DS. I have always let this go.
DS is now 17 months and the other day, DS fell over. DB was with him on the other side of the room so he picked him up quickly and passed him to my mum who was sat next to me. Two minutes later my dad referred to my mum as 'mummy' twice, instead of 'nana'.
He also kicks off if I do something for DS. For example. We were out for a meal and it was outside (in summer). DS dropped his dummy and dad picked it up and gave it to him. I gritted my teeth and didn't say anything, but he dropped it again and it was obviously getting dirty on the floor so I said 'oh don't give him that, it's dirty, I'll go and give it a wash'. Dad started rolling his eyes and huffing about it. Then he pulled a face and marched off to get a drink. They tend to let him run off quite a lot and if I go and get DS or ask them to keep a closer eye on him if they want to take him for a run around, dad gets really angry, and does the same rolling eyes etc.
He disregards what I say and double checks things with my mum for how they should be done.
I just find it strange. It's not something I'll bring up with him or anything, but it's like he refuses to acknowledge that I'm a parent.