Out of air with a full tank
A buddy of mine related this story. I've changed some details to protect her privacy.
A group of divers was out for a multi-dive boat trip in well preserved kelp forests just off the coast. Spirits were high and everyone was eager to get in the water. Our protagonist, Olga, had been looking forward to this trip for weeks and was diving with her daughter, some other divers on the charter, and a professional guide. They listened to the pre-dive briefing, quickly ran through their buddy check, and descended to about 80 feet. A few minutes into the dive Olga noted her pressure was 1500 PSI - still plenty to keep diving. The next time she checked however, her air-integrated computer was giving a critical warning that she was low on air with only 200 PSI remaining. Olga had fallen a bit behind her daughter and the guide but could still see them. Thinking quickly, she decided to swim to the guide where she signaled that she was low on air and needed to share air.
Olga successfully shared air with the guide, they ended the dive and ascended to the boat. Back on the boat Olga was safe but a bit shaken by what had happened.
What could have gone wrong here?! Everyone else returned to the boat with plenty of gas remaining.
Olga started the dive with a presumably full tank, and had a reasonable amount of air that seemingly vanished in just a few moments. We don't know for sure (because a full post-dive analysis wasn't performed by the charter boat or Olga) but the most likely explanation is that Olga's tank valve was only partially open. This is a common problem (we're all human and can forget to fully open a valve) that is also easy to avoid.
A partially open tank valve may read as full pressure when it isn't being breathed. It will supply gas to your second stage regulator for you to breathe, but only at a slow rate. When you are at the surface you might not notice that it isn't fully open. You don't consume much gas at the surface - so it may feel normal to breathe. At 80 feet however, your gas consumption due to depth alone (never mind if you are breathing harder due to exertion or excitement) is more than 3 times your gas consumption at the surface due to the ambient pressure of 3.4 atmospheres at 80 feet (you use 3.4 times as much air in every breath as you do at the surface). A partially open valve can easily fail to deliver adequate pressure at depth even if it breathed fine at the surface
Become a Better Diver By
ALWAYS completing a full safety check before EVERY single dive. As part of checking your gas supply (air or other gas mix) this should include making sure your tank valve is completely open. Take a few deep, relatively quick breaths from each of your regulators (you want to make sure your octo works too, right!) to challenge your first stage and make sure it is delivering plenty of air without a drop in pressure. Watch and see if the gas pressure on your gauge drops when you take a breathe. It shouldn't (at least not by much). If the pressure drops when you breath that may indicate a partially closed (common), malfunctioning (rare) or blocked (very rare) valve. Get it fixed before you dive (which can be as simple as fully opening the valve).
If Olga had performed this safety check, she likely would never have been "out" of air on this dive. She has since added it to her pre-dive safety check procedure.
Learn and practice how to fully open a partially closed tank valve while you are diving. Simply reach behind you and open it fully if you forgot to do so before your dive. In the event you unexpectedly run low on air (e.g. at the beginning of a dive) you can try this self-rescue maneuver while you swim your way over to your buddy to share air or while you prepare to self rescue from a pony bottle.
If your buddy is carrying your only alternate gas source on their back, stay close enough that you could reach them comfortably and start sharing air even if you had to make the swim without getting any additional gas from your tank. How far is that for you? For me, my comfort zone is a couple of seconds away - particularly here in the Pacific Northwest where low visibility means that if you're not close to your buddy you probably can't see them either. Check regularly, always know where your buddy is and if they are OK on gas.
Do like my buddy did, practice using an alternate gas source and if you find yourself in an out-of-air or low-on-air situation don't hesitate to use it. This is much safer than making an unplanned emergency ascent (CESA) to the surface particularly on deeper dives.
On deep dives (~60'+) consider carrying an independent alternate air source like a pony bottle. Calculate it's volume to ensure it would be adequate to allow you to ascend to the surface at a normal slow rate and complete a full safety stop. Just like with your other gas supply equipment, make sure it works as part of your pre-dive safety check for EVERY dive.
If you have a problem underwater - figure out what it was and fix it before you dive again. It would have been good if the dive charter, knowing Olga had unexpectedly run low on air, had checked to see if a partially open tank valve was the problem - or something else. Without knowing the problem you risk repeating it. Choose dive outfits and dive buddies who prioritize your safety and learn from and correct small issues before they become big problems.