It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].

Today’s kotowaza

河童の川流れ　Kappa no kawa nagare /Even a Kappa can drown

A kappa —also known as kawatarō, komahiki, with a boss called kawatora or suiko —is a reptiloid kami with similarities to yōkai found in traditional Japanese folklore. Kappa can become harmful when they are not respected as gods. Wikipedia – to win

kawa – a river

nagare – flow

　Kappa – かっぱ

Original

Even a good swimmer like a kappa sometimes drown.

Its meaning

Even a person who is good at something, will fail sometimes.

An example

Zenko chorei no toki, ongaku no Koizumi Sensei ga kouka no piano banso o machigaeta. Kappa no kawa nagare da!

When we were standing and singing a school song, a music teacher made an mistake playing a piano. I was surprised! Even a Kappa can drown.

arigato