That happens to me all the time. I swear I either have an amazing internal clock, or I am peeking at my clock in my sleep without knowing it, because I will wake up 5 minutes, even 1 minute before my alarm goes off. And I hate when something wakes me up like 20 minutes before my alarm and there's no point going back to sleep because as soon as I do, I have to wake up. >:I
I don't usually forget to set mine either but occasionally it slips by me. I have a back up, my cat. She usually bugs me and jumps on the bed all purring and lovey dovey when it is time for her to eat. The rest of the time it's "leave me alone or I'll scratch your eyes out!" So even if my alarm fails the cat will wake me.