Oh man, I love this thread...
Well, as long as we're all in confessional, here are my top three:
3) first time I cooked ribs, I was young, super impatient and rolled my eyes at why one would need to put the work/time in to manage a low-n-slow cook for 5-6 hours. I was proud that I got the internal temp of those ribs up to 190F in just over one hour and couldn't understand why they were so tough and chewy.
2) known as the grill master, I made the mistake of trying to grill twelve 14oz fillets at once for my MIL's guests --to order-- on my 22" kettle, using a dual zone. My MIL wanted me to time it so everyone would be served at the same time, which I didn't think would be an issue. As you can imagine, trying to manage that many steaks to order for the first time like that became quickly overwhelming when I was having trouble maintaining zone temps, doneness, and was getting flustered because it was taking waaaaaaay longer than I expected. Because of the need to serve everyone at the same time, the ones who liked it rare were happy, but I ended up bringing 10 fillets back at varying times to cook them properly. I still hear about it to this day (jokingly of course).
1) using my mom's huge gas grill that had a broken ignition system - nearly went full Hannah Storm and singed my eyebrows while manually lighting the grill because I thought I had waited long enough to let the first round of gas to escape before lighting. WHOOMPH!!