Someone held a button down for too long =)
Nigel, before you say you're sure of something and state that membrane keypads are unreliable, keep in mind they're used on a VAST array of devices, they're just not meant for extreme duty use, for a microwave the chances of one wearing out are VERY low, we had scales at work that used membrane keypads and they failed roughly every two years, but this was for a button that was hit 500-1000 times a day, and it was in a very caustic environment. Typical membrane switches are extremely reliable in the correct environment.