You may faced md5 mismatch error when you want to restore your nandroid backup that was created earlier using CWM recovery. The backup file will save by default into your sd card clockworkmod\backup folder.
The backup files inside backup folder are like boot.img, recovery.img, system.ext4.tar etc. and one file nandroid.md5 that is having information of all these files.
The cause of md5 mismatch error as I think so, you have copied the whole backup inside your PC earlier and again copied to sd card when you want to use. But the references of files inside nandroid.md5 has changed, so CWM recovery tool unable to locate the files.
The solution of md5 mismatch error –
Using some file explorer like ES file explorer inside your mobile delete this nandroid.md5 file. Now create a new bank file and rename it as nandroid.md5 thats it.
Now boot into recovery and try to restore backup it should now work successfully.
See the following posts more more info –