The Gemini launch vehicle tanking exercise, normally a part of WMSL, was conducted separately for convenience on July 17. SLD was a dress rehearsal to demonstrate the coordination required to conduct a single countdown on two vehicles and was subsequently performed on all rendezvous missions. The mission control centers at Houston and the Cape, as well as Eastern Test Range support facilities, were integral parts of the combined countdown. A failure in the Houston computer system caused several spurious commands to be transmitted to the target vehicle. Although some of these commands were accepted, results were not serious because they were mostly stored program command loads. Following SLD, the Atlas and Agena were demated on July 26.