1. Forgetting to watch the time and staying down too long.
2. Using air quicker than planned due to overshooting depth, getting caught in a current, getting lost, nitrogen narcosis, diving in cold water, getting caught in kelp or fishing line.
3. Sudden regulator malfunction due to corrosion or too much time between overhaul.
4. O-ring failure on any of the First stage regulator ports.
5. Regulator stuck in "free-flow" due to foreign particles or sand.
6. Regulator "freezing up" in cold water leaving you suddenly without air.
7. Stuck Pressure Gauge reflecting incorrect pressure in your tank.
8. Air cut off due to tank valve not turned on all the way.
9. Rupture of SCUBA tank o-ring due to misalignment or wear.
10. Second stage regulator diaphragm develops a tear or slips out of place.
11. O-ring failure on BC air inflator or valve stuck in the open position.
12. Burst disc failure on SCUBA Tank valve due to corrosion or age.
13. Clogged filter on first stage regulator due to sediment or rust from tank.
14. Leak develops at any one of the High or Low pressure hose fittings.
15. Mouthpiece on second stage suddenly coming off regulator.
16. O-ring failure on pressure gauge.
17. Rupture of either High or Low Pressure hoses.
18. O-ring failure on Octopus regulator.
19. Rental tank not filled completely.
20. Damaged First stage regulator due to falling object / impact.
21. High or Low pressure hose cut or severed on sharp rock / impediment.

(Source: Dive accident logs, published articles in dive magazines, and letters from SPARE AIR customers.)