Well done, Steve. We needed a 'confessional' thread, and you've started one.
Actually I'm going to disappoint. I can't recall any serious howlers. Had a scare while building 'Cadenza' (my Storm Petrel - a double-ender) when bending one of the strakes round to the sternpost - quite a heave, lots of tension (in the strake and me), and a loud resonant 'craaack' from the hull (not me) as something gave. I can't now remember what had cracked, but that, believe it or not, was in the days before emails-for-everyone, and instant digital photos (phew, how much the world has changed in the last ten years!!!), and I had to do sketches and then post (remember that?) them to Nick Newland, to be re-assured - by phone, that had actually been invented by then - and told what to do next. A tower of strength was Nick-on-the-phone.
What did happen during both my builds, however, was frequent ruination of perfectly good sweatshirts, fleeces and trousers by smearing them with wet epoxy. It was a matter of 'bother, I should have put my overalls on'; 'i'll just do this bit and then put my overalls on - drat, how did that happen?'; 'this job doesn't need overalls ' (it did);'now I can take my overalls off, and I can just check that if I lean carefully over - blast, now look what I've done'; etc.