[twic]

I don't know anyone who failed their viva entirely, but it's standard to be asked to make corrections to the thesis after it. Getting passed without corrections is a significant achievement. Those corrections can relate to weaknesses in your general knowledge uncovered in the viva: if there is something the examiners think you should know about, and you don't, you can be asked to add it to the thesis.

My impression was that someone failing their viva was seen as a serious failure on the part of their supervisor, who should not have let them submit without being more prepared.