I agree with you, a true friend would excuse a lapse, a forgotten birthday, and I'd feel a little annoyed if someone was to hold me to task over it. I remember the birthdays that I need to and this case I didn't forget, I just remembered the date incorrectly. A subtle difference maybe, but different nonetheless.

It was unusual to associate your girlfriend's birthday with the day Guy Fawkes tried to blow up the House of Lords

Not my girlfriend but yes, maybe unusual. I thought it was on that day though, and as a history buff it's a day I'm familiar with and knew I'd remember the birthday when it came around. The problem was I got the wrong day. Lol.