TL:DR I bought a card for my ex on behalf of our son and it's upset ex's Gf.
Been separated for 3 years, divorce being finalised any day now. He lives with his GF nearly 2 hours away, DC stays with them every other weekend.
In Sainsbury's a couple of weekends ago with DC (7) and he spotted a blank card with a picture on the front that was a play on his dad's name. Hard to explain, but it wouldn't be amusing to anyone not called by Ex's name (and wasn't that funny anyway but, being 7, to DC it was hilarious).
DC asked to get it for his dad, so we did; and he wrote a little message inside and drew a picture. Ex came to collect last Friday for the weekend and DC gave him the card, all giggly. Ex did appropriate laughy reaction, said thank you, told him he'd keep it by his bed with a picture DC had drawn for him recently. Thought no more of it.
This afternoon I've had an email from Ex, which is unusual because usually we communicate via text. Very formal, and says that he would appreciate it if I refrained from purchasing any more cards for him in the future because his GF "finds it a bit inappropriate". I immediately replied and said I hadn't got him anything; the card was from his son. Got back: "Yes but we agreed a couple of years ago that GF would purchase all gifts and cards on DCs behalf for Father's Day, birthdays etc. so I think GF felt that this wasn't quite sticking to that arrangement".
I feel completely 'Wtaf' about this but I'm in a baaaad mood today anyway and I have a habit of instinctively reacting to things and then calming down and realising I might have misread the situation. So, in the spirit of trying to rein myself in just in case this is one of those times, please tell me if iabu? I'd particularly like to hear from anyone who is with someone else's ex. I've never been in that situation and I don't know, maybe this was wrong somehow?! And if I'm not BU to feel 
about this, what do I respond with?
Have nc'd because I'd rather this wasn't linked with my usual name.