An apparatus 2 comprises at least three processing circuits 4 to perform redundant processing of a common thread of program instructions. Error detection circuitry 16 is provided comprising a number of comparators 22 for detecting a mismatch between signals on corresponding signal nodes 20 in the processing circuits 4. When a comparator 22 detects a mismatch, this triggers a recovery process. The error detection circuitry 16 generates an unresolvable error signal 36 indicating that a detected area is unresolvable by the recovery process when, during the recovery process, a mismatch is detected by one of the proper subset 34 of the comparators 22. By considering fewer comparators 22 during the recovery process than during normal operation, the chances of unrecoverable errors being detected can be reduced, increasing system availability.