mock.rb

# Original mock syntax, straight forward example
Object.any_instance.should_receive(:foo)
Object.new.foo # satisfies mock

# Original mock syntax, confusing example b/c of "any" term
Object.any_instance.should_receive(:foo)

Object.new.foo # satisfies mock
Object.new.foo # error, "another instance has already received the :foo message"

# Proposed syntaxes
expect_any_instance_of(Object).to receive(:foo)

# ... or something like ...
expect_an_instance_of(Object).to receive(:foo)
expect_one_instance_of(Object).to receive(:foo)

stub.rb

# Original stub syntax, straight forward example
Object.any_instance.stub(:foo).and_return(:bar)
Object.new.foo # => :bar

# Original stub syntax, confusing example b/c of "any" term
Object.any_instance.stub(:foo).and_return(1, 2, 3)

o = Object.new
o.foo # => 1
o.foo # => 2

Object.new.foo # => 1 (NOT 3)

# Proposed syntaxes
allow_any_instance_of(Object).to receive(:foo).and_return(:bar)

# ... or something like ...
allow_every_instance_of(Object).to receive(:foo).and_return(:bar)

expect(Object).every_instance.to receive(:foo)

expect(Object).an_instance.to receive(:foo)

Though the an_instance is still confusing in the case that you create multiple instances.