Mistakes I have made over the years has to do with equipment failure! Not checking that all is functioning as much as I should have! Also acclimation. When I first started my attempts at ceph keeping, I would be so excited I would rush the process resulting in shock and usually death within 24 hours. The slower the better results! Not escapeproofing your lid is another! Ducktape it to death, even if they have never attempted to come out, there always is the fatal first possability, we didn't think they could squeeze through some little hole that was overlooked!