I lost a trainee builder last night - there were two other builders working on the idol and I dragged over another guy to train him - but he disappeared. His details were still there because I'd had him selected, but when I selected someone else, his details disappeared.

I was on level 2 for medicine, fertility and science and level 3 for the others, eight puzzles completed (still waiting for the GC) and had a population of about 26 having just finished the third hut.

I dropped him at the bottom left-hand corner of the scaffolding when he did his "thin air" trick. Not a major loss for me, but it might help you pin down the bug.