I lucked out on the birthdays. Mine is on May 1st and his is on May 2nd. So we remind each other and then have a special outing for "our" birthdays. If we buy presents, he can't wait to give me mine. So I get my present within ten minutes of him walking in the door. And I'll hide his present in plain sight. He misses it every time until we exchange them.
If either of us forgot a birthday, it would be both of us forgetting at the same time. I don't see that happening.
That's funny since my mom and dad were a day apart too, so he never forgot, although he wasnt' great about remember other milestones!
I had a situation once with an ex-BF; we were in a long-distance relationsh*p, and had been together for several years. My birthday came and went, and I thought, oh, he's in grad school, and doesn't have much money, and he DID call so I figured I wasn't getting a gift, and that was ok.
Next year rolls around, adn I commented something along the lines of how it was ok he hadn't gotten me anything the previous year. He then said, what do you mean, I sent you flowers! Which I never got. And he never bothered to ask me THEN about them when I never thanked him! I said you know me better than that and KNOW that I'd never not thank you, so you shoudl have gotten a clue, and had you said something, we could have dealt with this then, but now its too late! Still don't know where they went!