devtool modify/upgrade set up an oe-local-files subdirectory of the source tree to provide a place to make changes/additions to non-patch local files, however it is possible that the user will not realise this and attempt to modify files next to the recipe - in the case of devtool modify these would be the original files next to the recipe, in the case of devtool upgrade they would be in the workspace next to the upgraded recipe file. If the user does modify these files then nothing will actually happen - the files in oe-local-files will be used in preference. If practical we should at least show a warning if those files are newer than the ones in oe-local-files; bonus if it ignores this if the checksum has not changed from the original. The practical question though would be when we would do this checking and warning; presumably it would have to be triggered within the metadata or bitbake to be useful.

(See also bug 13079 - part of the reason why users might miss this is it's currently undocumented.)