…but it doesn’t mean something is wrong with you or wrong with the viva.
You go blank and it’s uncomfortable: doubt, anxiety or fear. It’s not that common an occurrence though.
In the unlikely event it happens, breathe. Pause. Take a sip of water. Ask yourself the question again, or think about what’s just been said. And whatever you need will come to you.
A momentary memory lapse or glitch is not comfortable, but it’s not the end of the world, even in the viva.