Ask students to identify a bug in code. You can model good debugging practices by showing various inputs that produce correct and incorrect output. You can ask students to fix a bug that you demonstrate with tests or ask them to find a bug. For example, you can ask students to write a test case that will demonstrate the bug.
Example: "Programming involves a lot of debugging. Let's practice!"
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-assessing-programming