И на старуху бывает проруха

What exactly does this phrase mean?
What is “проруха”? And how should it be pronounced?

“Проруха” is an archaic word that means a mistake, blunder, foolishness.

The phrase means that anyone can make a mistake; nobody’s perfect.

It literally means that even an old lady (i.e. an experienced, wise person) can make a mistake.
Here go some English counterparts of this saying:

  it could happen even to a bishop!

  accidents will happen in the best regulated families

There’s another saying which is very close to it in meaning:

There are spots even on the sun / На солнце тоже есть пятна

Example:

  Авторитет этого режиссёра считается непререкаемым, но, как говорится, и на старуху бывает проруха. Его последний фильм подвергся резкой критике жюри, которые нашли его работу крайне неудачной.
—————-
This director is supposed to have indisputable authority, but, like they say, there are spots even on the sun. His latest film was fiercely criticized by the jury who found his work extremely disappointing.