I'm running gmusicbrowser 1.0.2 on Fedora 12 x86_64 (installed from the official Fedora repository).
After a few hours of normal playback (of FLAC and MP3), deleting some files through gmusicbrowser (right click -> "Remove from disk"), and deleting some files normally (rm -r SongsIDontLike), gmusicbrowser stopped with this error:
Playing error : Resource not found. at /usr/bin/../share/gmusicbrowser/gmusicbrowser_gstreamer-0.10.pm line 135.
The program is still open (I haven't clicked "Close" on the error window), so if there's something I can check, let me know.
If you deleted files manually, it's probably simply because the file you're trying to play doesn't exist anymore.
If it's not that, it's possible that gstreamer encountered an error while opening the sound output, maybe a problem sharing it with other apps.