You can try to verify with (if smartmontools available and working for mtd block devices?):
smartctl -a /dev/mtdblock1
The tool will tell you what kind of error it is. Google will tell you you if it is reapairable and if it is harmfull or not. Usually those errors are not harmfull and just deleting the sector will fix it. debugfs will tell you if the sector is empty and safe to delete e. g.: debugfs /dev/mtdblock1. You can delete/zero with dd the affected sectors.