My own personal "staying awake at night" case is an application that connected to an ancient version of Banner (A big kludge of an ERP system for universities) to handle new applicants' enrollment process and billing. I'm rather skittish when directly handling money, doubly so when the code was all written in Perl (Which I had to learn in order to re-implement in far more readable PHP, the lesser of two evils, natch) and extremely poorly documented. In retrospective, I should not have accepted a job like that.