My 8th - twofold. Firstly, one of the dogs I'd grown up with had been euthanised the night before (stroke and heart attack, aged 12, GSD, relatively good age), and I'd been there when the vet said "no, I'm sorry, there's no hope of any recovery". I spent the day of my birthday apologising to my school friends about my father's emotional state (it was, technically, "his" dog). Secondly, my parents had decided to throw me a whole year birthday party. And surprise me with it. It freaked me the fuck out. I don't "do" crowds, as a general rule, and I didn't like half the kids there (it's okay; they didn't like me either!).
My 18th - I broke up with my boyfriend at the party, 4 weeks after he had literally tried to strangle me. I spent most of my party (another "let's surprise 13!!!" bash) sitting in my bedroom with my back against the door, my best male friend sat alongside me, counselling me. He later punched my then-boyfriend, which didn't help. I also ended up meeting and subsequently getting together with my daughter's biological co-creator that night - after he'd literally gate-crashed my party...
My 28th - spent trying to gird my loins for my best friend's wedding four days later. I was 37 weeks pregnant at the time and had to travel from Surrey to Wales, with an 8 year old in tow. I was also on the verge of splitting from my son's father, because he'd decided not to accompany us to Wales, despite my saying that I might go into labour there (I didn't). It was the "... and?!" attitude that killed any respect I might have had for him as a father - although he was a great stepfather to my daughter. I gave birth 2 weeks later, to the day, and again, my ex pointed out to me how useless he actually was (he went for a nap in his car, because he "was exhausted" after 5 hours of labour...).
My 40th - I was supposed to be visiting friends in Dublin for a childfree weekend. My mother, upon being asked to childsit for 2 days, literally invited herself and my children along... so I cancelled it. I spent it, instead, being ignored.
I've also not had a cake since my grandmother died, when I was 22. It hurts, because I always try to make birthdays special for those I love. But it is, what it is.