I once did the "prep all weekend, 3 flights to nowhere and drive to dirty warehouse to start due diligence" trip, only to show up and discover (to my delight, actually) that I was allergic to the client and would have to turn around and go home. The investment bankers had forgotten to tell my law firm that the client (which manufactured penicillin-based drugs) had prohibited the staffing the deal with any junior staff from the law, banking and accounting firms (ie those who would be crawling around in the warehouse or handling its documents) who were allergic to penicillin. Which I was. So, a long trip for nothing (except the miles) - but it kept me out of the office for a little while and I managed to avoid some other work because people thought I was busy on the bacteria file.