have feet of clay
US /hæv fiːt əv kleɪ/
UK /hæv fiːt əv kleɪ/

1.
to have a hidden flaw or weakness in one's character, despite outward appearances of strength or integrity
:
•
The public was shocked to learn that their revered leader, who seemed so perfect, actually had feet of clay.
•
Despite his impressive resume, it became clear that the new manager had feet of clay when he failed to handle the first major crisis.