When a small or medium memory model program exits, a check is made to determine if the contents of the first few bytes within the program's data segment have changed. These bytes would never be altered by a working program. If they have been changed, this message is displayed to inform you that (most likely) a value was stored to an uninitialized pointer.  

The program might appear to work properly in all other respects; however, this is a serious bug which should be attended to immediately. Failure to correct an uninitialized pointer can lead to unpredictable behavior (including locking the computer up in the large, compact, and huge memory models).  

You can use the integrated debugger to track down null pointers.