No doubt that's inexcuseble BUT realize that like you, auto techs are human and do make mistakes. Things also occur in shops (phone rings, another customer walks in, boss interupts, etc) where the tech is distracted and thus might forget where he left off. I'm not trying to make up excuses and yes, a good tech should always double check his work making sure that everything is tight but in my years, I've seen many a "tech" make bad mistakes.