My mum had a big birthday recently and for part of her present DH and I offered to take her out for a fancy dinner.
Think high-end £100+ ish a head type place. Not the sort of place we would usually eat at but, as I say, it was a big birthday.
There were six of us; me DH and our two DC; my mum and her husband. When the bill came I had expected my mum's husband to chip in and cover his share, leaving me to pay for my mum and the rest of my family... but he didn't. He had assumed that me treating my mum meant me treating him as well.
Was I in the wrong here for assuming that me offering to treat my mum meant covering her dinner and not his too? (And yes I should have been more explicit; I know that now! I just paid the whole thing and smiled rather than have any drama at what was otherwise a very nice family meal out)