The same is true if you accidentally leave your unindented code in... Except you can't turn it off on Prod as quickly as using a constant/variable to control it. Personally, I'd even go one step further and create an Environment variable so you can use $_ENV and have the ability to turn it off across the entire server. The issue is purely the fact that the test/debug code is being left in. The real question is, which method has less of an impact if that code made it to Production.
Unindenting it won't necessarily prevent an accidental checkin, neither will TODO or HACK comments (even if you are always checking for them before checkin) as at some point you'll get distracted and just check it in (happens to everyone). So every method has issues (to assume otherwise, is negligent).
In all reality, the best way to introduce debug/test code is DON'T DO IT. Use xDebug, set a breakpoint, profile it, don't add random code that provides nothing to the codebase.