The launch of US Air Forces AEHF 2 communications satellite was postponed on 3 May due to a problem with the helium flow during the purge of the Centaur interstage adapter prior to launch. This was most probably caused by a misconfiguration of a manually operated valve. Launch has been rescheduled for 4 May with a launch window of 1842 – 2042 GMT.
Or in other words, someone forgot to switch a tap on.