Reasons side effects are not bad but evil.

- Having a side effect means you are always working, testing, and thinking about more than one unit
- They often break one or more of the following principles SRP, Law of Demeter, Encapsulation, etc
- Creates known & unknown implicit dependencies
- Side effects can also have side effects (ahem... ActiveRecord callbacks)
- Causes more code churn. Changes in one area cascade to other areas.
- Makes test setup harder. Your test setup contains objects that are seemingly unrelated to the code under test.

Alternate titles:
- Learning to Shave a Yaks while Programming
- Side Effects are like CSS for the rest of your code; all changes cascade.