On my last dive I had to ascend from 60ft through a swarm (is that the right collective term?) of jelly fish.
My buddy didn't follow me, smart guy. He stayed on the outside and took pictures. Viz was terrible as we reached about 40ft, so during the ascent I lost sight of my buddy.
None of this would have been a problem normally, but the reason I drifted into the jelly swarm, was a brief spot of distraction as we started the ascent. As I looked at my dive computer to monitor my ascent rate, I noticed all the digits were ... how can I describe it ... doing cart-wheels. No depth reading, no air reading, nothing. Mild panic. I pulled out my console , which has my old air gauge - at least that still showed I had air in my tank, though only 500psi.
Any one of these issues alone and things wouldn't have seemed so bad. Low on air, n.p., buddy separation, n.p., no depth gauges, n.p., but all at the same time along with the jelly ... very annoying.
I had to simply follow my bubbles back to the surface ascend slower than your smallest bubbles. in poor viz and open water you might be ascending slower than your smallest bubbles, or you might equally be descending.
It was a case of bad timing - everything going wrong at the same time.
One thing I am going to fix before my next deep dive is to ensure I have a backup computer. Conversely, one piece of excellent timing is the fact that Oceanic have recently come out with the B.U.D. Dive Computer. A low cost, back-up dive computer that is intended to be attached to your dive gear. As Oceanic says:
B.U.D. is a long overdue idea and a great way to ensure that you never have to miss a dive. Clip it off or stow it in a pocket and forget about it. You should never have to use it, but if you do need it, your B.U.D. will be there to save your next dive.
See it at Amazon.