It is a well known rule that assert statements should cause no changes in state outside of them (aka side effects). Until now, you could only check if this rule was followed by visually inspecting the code. In IntelliJ IDEA 9 a new assert statement with side effects inspection will be available to help you keep assert statements free of side effects.
The inspection will warn about modification of variables and fields inside of the assert statement. The assert statement will also be reported if a method called by it modifies any fields.