An exemplary fault-tolerant computing system comprises a secondary processor configured to execute in delayed lock step with a primary processor from a common program store, comparators in the store data and writeback paths to detect a fault based on comparing primary and secondary processor states, and a writeback path delay permitting aborting execution when a fault is detected, before writeback of invalid data. The secondary processor execution and the primary processor store data and writeback may be delayed a predetermined number of cycles, permitting fault detection before writing invalid data. Store data and writeback paths may include triple module redundancy configured to pass only majority data through the store data and writeback path delay stages. Some implementations may forward data from the store data path delay stages to the writeback stage or memory if the load data address matches the address of data in a store data path delay stage.