Wow, most of these stories leave me feeling like my "worst dive ever" was far more fun than what I thought it was!
I'm going to go with my first dive after OW cert as well... We actually signed up for and did the PPB class right after our OW dives...
After learning about weighting a bit more in-depth (pardon the pun), I ended up dropping about 10#'s of lead, when I really should have only dropped 5#. We were using our 1/2 full (or 1/2 empty depending
) tanks from our last OW dive that morning, so we started out with around 1500 or 1800 Psi. After about oh 20-30 min (I don't remember and I don't have my log book handy) I was starting to get down to around 700 or so PSI and that's when I started feeling "light". My wife and I were on the platform, I had finished my skills and she was with the instructor doing her skills when all the sudden I felt myself floating up a bit more with each breath.
I dumped all the air out of my BC and for the next 3-4 breath's I was fine, but then I started to float again... With each breath the tank got lighter, and I went up more. By the time I hit 15' I was head down kicking to stay down. Luckily the DM for our dive was paying close attention, and I gave her the big THUMB, we surfaced together and waited for the instructor and my wife to surface (only about 5 min later, but felt like forever).
The worst part of this dive however, was that another instructor was following us around (unknown to me at the time) because he wanted some more time on his rebreather. When it came to the part when I was head down kicking, I accidently smak'd him right in the head (hey 3' viz and I'm staring at the bottom).
I felt like a total idiot after that dive, and must have apologized 10,000 times too him. Lucky for me, he's a great guy, and he turned out to be our instructor a year later for our Nitrox cert! Amazing who you "bump" into underwater isn't it?