I've missed school from sleeping through my alarm tons of times, especially lately when I often only sleep from passing out from exhaustion after being up too long. (Whee, insomnia!) But luckily, I've never missed anything overly important (presentation, exam, etc.) from sleeping in. I tend to either forego sleep when I know there's something important in the morning and I don't trust myself to wake up for it or I'll set like 6 alarms on multiple devices in the hopes that one of them will wake me up. I'll also ask my mom to give me a wake-up call, haha.
Most important thing I've slept through was probably a doctor's appointment. It wasn't with a specialist or anything, thankfully, but ironically it was with my family doctor to get a refill on my prescription for sleeping pills. :P