Here's something that didn't seem dumb at the time, but in filling out my dive log later, I realized was extremely dumb... It was my last day on the 4th and final day of diving in the Bahamas, and my 8th dive since OW cert (so 13th overall dive).
Since it was just myself and wife (non-diver) on vacation, each day I had to be assigned a new "insta-buddy". Quite a range of buddies, but that's another story. On this particular dive, I actually joined a group of 5 other divers who were all friends and agreed to let me tag along. In hindsight, I should have requested a different situation, but that's not the dumbest thing.
The dive was to explore a coral reef in perhaps 45' of water. Excellent visibility and great conditions, with a slow current. This group didn't do any kind of buddy checks, etc. - they just basically jumped in the water and off they went. In hindsight I should have picked one of them and had them do a buddy check, but that's still not the dumbest thing.
So this group gets underwater and they are not really paying attention to me at all. They are also not really moving very much or doing much exploration of the reef - they seemed more interested in clowning around in front of a camera and trying to "photo bomb" each other. Meanwhile I see a school of jacks and decide to swim amongst them. In a short amount of time I find I'm separated from the group. But I feel really comfortable diving, correctly weighted, with my own equipment, etc. So I end up saying to myself, "screw those guys", and off I go 'solo' exploring the reef. From time-to-time I look around, locate the group and their bubbles, as well as others from the boat, so I figure everything is cool.
Well time is approaching where the DM said for everyone to be back. I look around and see no one - no bubbles, no boat, no anchor line. So I hover over a coral head at 15' doing my 3 min safety stop, still with about 1000 psi left. Finally surface and find I'm maybe 100' from the boat - no problem, I just swim over, hand crew my fins and climb up the ladder.
It's then I realize I'm the last one aboard; everyone else has been back at least 10 minutes, and everyone is already out of their gear and most out of their wet suits. Then crew member comes over to have me initial the clipboard, and of course I'm the only one left to sign. One of the guys from the "group of 5" says something to the effect, "hey man you didn't stay with the group." I replied, "sorry, I got distracted looking for eels" (which was partially true, but I also made the decision to not stay with this group). No one else said anything to me and we headed back to the dock, where I gave the crew a tip, rinsed my equipment and left.
Later I realized how dumb that was to leave the group for at least two reasons. First, if something did happen, for instance, suppose I got stuck in some coral, or got really far from the boat, I'd be in a dangerous situation real fast. Second, I must have really caused a great deal of concern to the boat crew, who has everyone checked off their list except one person! Thankfully they had that checklist (think of the movie Open Water - LOL). Anyway that was DUMB, DUMB, DUMB - and if any of your read and recognize this: my sincere apologies.