A problem was discovered very late in the development of the TXE4 in that if an equipment number was threaded with the wrong directory number on the cyclic stores, it could clash with the directory number of another equipment number leading to multiple directory numbers. This would cause Number Unobtainable (NU) tone when the incorrectly threaded number was dialled and the double threading number received incorrect calls. The exchange had no way of detecting this but the programming of one of the MCUs allowed a separate program to run to detect errors and print out the locations of the duplicates. This had to be done on a regular basis. Eventually, after many more diagnostic routines had been added, this became known as Tester 299A.