A defect is a critical bug that would result in a false positive result. For DBAN, this means a "green blinky" and corresponding "pass" log entry for a disk that was not actually wiped.
Notwithstanding driver problems, there are no known defects in these stable releases:
Stable releases are added to this list over time.
Some 9 gigabyte Seagate and Quantum SCSI disks, and the Fujitsu MAJ3091MP, report more sectors than are actually on the disk. DBAN treats these disks as defective and will abort at the end of the first pass. You can check whether you have one of these disks by booting with a DOS floppy and writing to the last sector with a hex editor.