Read-after-write
The verify technique where a read head sits just behind the write head and checks every block the instant it is written.
On a tape drive you cannot go back and check later without rewinding — so the read head is placed downstream of the write head and reads each block back in the same pass. If the check passes, the tape keeps moving; if it fails, the block is rewritten further along and the defective spot skipped.
The technique catches media errors before they become data loss and is standard on everything from old half-inch tape to DLT and LTO. The cost is one extra head and a little tape length — cheap insurance when the tape is the last copy of something.