Most skydivers are switched on when it comes time to pull. I only know of one where a guy pulled late and he was a student. It's on Youtube as it happens. In fact he pulled late but the Cypress fired anyway. His excuse was that he was 'Evaluating the situation'. I watched from the air, (I'm a Skydive pilot). I seriously thought it was a fatal one.
Headown or not, there is no excuse. Most skydivers have a gadget that beeps like crazy when they're getting low. They don't have to check their alti any more.
I had a Cyres fire on me once. I was in the back on a ferry flight wearing a student rig as is legal when you transit on a parachute aircraft. I'm not a skydiver, Half asleep, I was in the back. The boss was flying. He forgot about me and when someone jumped over the field. He stuffed the nose down and bang, I was woken up with a start as the Cyres fired. He had a hard time living that down and it cost him a few bucks. My main problem was that it disturbed my sleep. Being a skydive pilot is hard work with long days. I need my sleep.