Once upon a time I had to track down an issue where very rarely, with no discernible pattern the web app would produce garbled PDFs. We would restart all servers, everything would be fine for a month or two, then random bad output. Turned out this happened when an admin account remotely connected to an app server, which caused a reset of default screen resolution (only for admin accounts, not regular remote connections), which messed up the PDF library that relied on a specific resolution (it was HTML-to-PDF conversion). Lived with it for a couple of years at least before tracking down the root cause (after many many failed attempts). The fact that only the server with the reset resolution caused the problem confounded the issue.