The issue: You are using stub_const to change a constant value for your test.

stub_const "SomeClass::CONST", 'test'

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All of a sudden, tests fail with undefined method 'some_method' for #<SomeClass:0x00000000101433a8>.

The reason

When using stub_const before the Class containing the constant has been loaded, a module is automatically created with the name.

Since RSpec does no autoloading, it will create a SomeClass module by itself. This is arguably a good idea.

As a workaround, use stub_const in your Rails specs like this:

stub_const "#{SomeClass}::CONST", 'test'

This will invoke Rails' autoloading and fix RSpec's behavior for you.