RSpec allow/expect vs just expect/and_return
See the classic article Mocks Aren't Stubs. allow makes a stub while expect makes a mock. That is allow allows an object to return X instead of whatever it would return unstubbed, and expect is an allow plus an expectation of some state or event. When you write
allow(Foo).to receive(:bar).with(baz).and_return(foobar_result)... you're telling the spec environment to modify Foo to return foobar_result when it receives :bar with baz. But when you write
expect(Foo).to receive(:bar).with(baz).and_return(foobar_result) ... you're doing the same, plus telling the spec to fail unless Foo receives :bar with baz.
To see the difference, try both in examples where Foo does not receive :bar with baz.