## foo.rb

class Foo < ActiveRecord::Base
  validates_presence_of :number
  validates_uniqueness_of :number
end

## foo_test.rb case 1 (runs fine with no failures)

class FooTest < ActiveSupport::TestCase
  should_validate_presence_of :number
end

## foo_test.rb case 2 (2 assertions, 1 failure: "Can't find first Foo")

class FooTest < ActiveSupport::TestCase
  should_validate_presence_of :number
  should_validate_uniqueness_of :number
end