Ok, my bad. This is the line in my project, which broke, now raising private method '`' called for Object (NoMethodError).

I tried these also, which don't work in mirb, but do in irb:

> `echo 10`
private method '`' called for Object (NoMethodError)

module MyModule
  def self.test
    `echo 10`
  end
end
> MyModule.test
private method '`' called for Module (NoMethodError)

class MyClass
  def test
    `echo 10`
  end
end
> i = MyClass.new
> i.test
private method '`' called for MyClass (NoMethodError)

If I check MyModule.private_methods or MyClass.private_methods, both of them have backtick defined, but I can't call it, even within them.

The only way I could figure out to get it working with backtick private was:

Kernel.send(:`, "echo 10")