The researchers also found several engineering issues:
* The design did not have any hardware interlocks to prevent the electron-beam from operating in its high-energy mode without the target in place.
* The engineer had reused software from older models. These models had hardware interlocks that masked their software defects. Those hardware safeties had no way of reporting that they had been triggered, so there was no indication of the existence of faulty software commands.
* The hardware provided no way for the software to verify that sensors were working correctly (see open-loop controller). The table-position system was the first implicated in Therac-25's failures; the manufacturer gave it redundant switches to cross-check their operation.
* The equipment control task did not properly synchronize with the operator interface task, so that race conditions occurred if the operator changed the setup too quickly.[clarify] This was evidently missed during testing, since it took some practice before operators were able to work quickly enough for the problem to occur.
* The software set a flag variable by incrementing it. Occasionally an arithmetic overflow occurred, causing the software to bypass safety checks.