First off...great program! The best tagger I've ever used.
The 'Extract Cover...' option will default to a filename of folder.jpg. If a file with that name already exists in the selected directory, the program will prompt with a dialog titled, "Confirm Save As" that says, "folder.jpg already exists. Do you want to replace it?" When Yes is selected the file will not be overwritten if it has the hidden or read-only attributes turned on...no error is reported.
I don't believe that the file write should fail if the file is set to hidden and an error message if the file write fails would have saved me a lot of time.