When working with fixtures, what should be done to break the tests?

Question

Ruby on Rails

The application is tested with fixtures and adding a new user can break tests where the number of users is strictly specified.
There are a lot of tests to run them all every time.
How to write tests working with fixtures and be sure that they won't break anything?
rspec.

Answer 1

The answer is obvious, do not use code in which something is "hard-coded". Describe a couple of cases to give specific advice...