Please explain.

The Monkey King saw it and regretted it. He said, "It's all my fault." With a sigh, he stamped his foot and shouted, "Alas, I beat geese every year, but I didn't expect to be pecked by goslings in turn today."

I can tell right from wrong with my eyes, but I didn't expect to see it today.

As the saying goes: I've been fighting geese all my life, but now I'm pecked by them.

It means that I have experienced countless storms, but now I have capsized in a small river ditch.