In Cards on the Table (S10E02) Hercule Poirot states:
The question is, can Hercule Poirot possibly be wrong?
No one can always be right.
But I am.
Always I am right.
It is so invariable it startles me.
Was that right? Has there been any instance, where he made a mistake?
I have seen only a few episodes, and I cannot recall any such occurrence.