Certified about 20 years ago. Started my instruction on the Great Barrier Reef and wrapped it up in Maui on the way back home. We dove some spot off Lanai called the "Cathedral", a volcanic room that let's a small piece of sunlight in from an upper chamber that shoots to the sand on the bottom. Pretty cool.
What wasn't cool was my final session. This is where take your mask off, put it back on and blow out the water with your nose. Within about a minute of going down I did that. Only half the water came out, so it was still at eye level. Did it again and it barely moved. Instructor gave me the OK signal and started coming toward me. He grabbed my shoulders, looked at my air gauge. Eyes got really wide, like when Dreyfus pulled the tooth in Jaws. We started ascending and I suddenly realized I couldn't breathe, so I pushed him the fuck away and went for the surface. Luckily we were in probably only 30 feet of water off Lahaina harbor, so I made it pretty quick.
Got to the surface and he yelled, "You're out of air!" The boat was within shouting distance, he said the same thing. "He's out of air!" His girlfriend, responsible for giving me an empty tank, put her hand to her mouth and probably muttered "oh my god."
The jabroni tried to spin it as if it was my fault I didn't check the tank before getting in, while alternatively apologizing profusely.
PSA always check your tank before getting in.