I have several servers to test new code on. I primarily push out asp.NET web applications. Last week, I had an issue where I installed a newly developed web application on three servers. The three servers all run in separate environments. The application worked fine on two of them, but consistently crashed on the third server with each web request. The problem was eventually traced to an in-house developed .dll file being out of date on the third server.

I'm certain that this kind of thing happens all the time. However, there are numerous things that could go wrong to cause this kind of behavior.

I spent quite a bit of time tracing this problem. I would like to make a list of things to be suspicious of next time this happens.

What are the most likely reasons that a web application would crash on one of my servers while identical code runs fine on another server?