As a prosecutor, you have absolute immunity when you're doing things that are in the "shades of gray" territory -- the kind of slightly-unsavory stuff you see on a regular basis on Law & Order.

If you commit a crime, though, your immunity is gone. Suborning perjury (as in, working with a witness who plainly knows nothing, training them to tell a consistent, and entirely-made-up, story), or introducing evidence you know to be false, violates your oath as a public servant. As such, you give up the shield of immunity that your service provided.