A bad dive for me would be one you didn't return from. Hopefully you learn something even on dives when something goes wrong.

Perhaps my worst was the day I decided not to strap on my pony (I'm a solo diver 95% of the time) because my dive "plan" was to go to 40 ft. I should have known better since my "plan" is often scrapped as soon as I see something interesting to film. I ended up following a bat ray down to about 75 ft. I had just exhaled and tried to draw another breath of air from my reg, but nothing came through it.

Instinctively I started to ascend slowly as I checked my gear. SPG read zero, no air in BC. I did a CESA from 70-75 ft and was rather breathless when I surfaced. No more diving for the day. Apparently debris got trapped in the dip or debris tube in the tank valve when I descended head first.

Lessons learned? Always keep my pony bottle on regardless of my "plan." Descend horizontally rather than head first.