If you have a recipe applying patches (via SRC_URI += <patch file>), build that recipe at least once, then move the patch file to a second valid location (e.g. from <pn>/<patch file> to files/<patch file>), bitbake will spit out the following warning: WARNING: Unable to get checksum for <PN> SRC_URI entry <patch file>: [Errno 2] No such file or directory: '<path to patch file's previous location>' The only way to clear the warning appears to be wiping out your parse cache, e.g. rm -r tmp/cache. Rebuilding from that point on will make the warning go away. I'm believe the patch is still being applied even when the warning appears (though I am not certain), but the message might lead you to believe otherwise. This warning should probably not appear if the patch is found in a second location.
Patch sent to the bitbake-devel mailing list to fix this. FYI, you can work around this problem by running "touch" on the recipe file - no need to delete the cache. You are correct in that the patch file is being applied even when the warning is shown; all that breaks is the calculation of the checksum which we use to trigger a re-run of do_fetch and all tasks that depend upon it when one of the local files referred to in SRC_URI changes.
Patch merged to master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=be78283e6cd7bea2eb1a5c5b797f1a6315468da5