Version:           1.5.0 (using KDE 4.5.3) 
OS:                Linux

If you have read-only files on your harddisk (e.g. because they have been copied from a CD ROM), Digikam obviously can't write to this files.
If you are editing these kind of pics, you will notice that, because digikam issues aa warning. 
But if you are editing only the metadata of the files (geotagging, adjusting timestamps, EXIF rotate etc.), these data will only be written to Digikam's database and not to the original files without any notice to the user. This leads to inconsistencies between the EXIF metadata and Digikam's database, which is especially annoying when also using other programmes or platforms with your photos.
IMHO Digikam should also issue a warning if you change only the metadata of a read-only picture to inform the user that his/her adjustments will not be saved in the picture.

Reproducible: Always