A key reason for the version incompatibility is the structure of the DLL file. The file contains a directory of the individual methods (procedures, routines, etc.) contained within the DLL and the types of data they take and return. Even minor changes to the DLL code can cause this directory to be re-arranged, in which case an application that calls a particular method believing it to be the 4th item in the directory might end up calling an entirely different and incompatible routine, which would normally cause the application to crash.