A method, apparatus, system, and signal-bearing medium that in an embodiment issue a warning if a file to be used is an older version. In an embodiment, the warning includes an identification of the location of a newer version of the file. In an embodiment, the file is a class, and the old and new versions are found using a classpath, but in other embodiments any type of file or other object may be used. In this way, the use of incorrect versions of files may be detected and avoided.