Software but a nice bug: A really long time ago I worked at a company creating a nice portal application in ASP.NET (version 1.1) for a client. Was cool to build. Client did not follow our guide on how to install the application. We tolled them it should run on a separate machine and they just crammed it together with 7 other web applications. Since the portal had a login feature where people could change their resume with was quite sensitive. At the certain time we got a call that people saw each others resume. We used most desktops in our office to simulate the issue and wrote scripts to simulate different users. it took weeks to finally reproduce it. As it turned out it was not our problem but a bug in ASP.NET... we had lots of calls with multiple offices of Microsoft. At some point we heard nothing back from them. We wrote our own state manager to avoid the issue but that also did not solve it. A few months later .NET 2.0 came out. One of the item in the release notes was that a fix was made that when too many requests on a ASP.NET server (IIS) would make the http.dll (not sure on the name) serve a cached version of the previous request.. We lost the account and a 100K of work that was never paid and we almost wend to court on this one...