As with the seven deadly spiritual sins, you can find someone who will argue that each of them is the worst of the seven. We won't argue that any of the coding sins is the worst, but we do believe Bugs and Potential Bugs is the most urgent. That's because this sin represents things that are going wrong in your code today or that may go wrong tomorrow. An unconditional null pointer dereference is a prime example of a bug (hopefully the urgency is self-evident). Potential bugs are a bit more subtle, but no less important.
Instances of this sin are tracked with the issues mechanism. Typically bugs and potential bugs will show up as Blocker or Critical issues, although that's fully configurable. No Coding Standards infractions are also tracked as issues, although typically at lower severities. Instances of the other sins can optionally also be tracked using the issues mechanism (which offers some distinct advantages).
To monitor potential bugs, add the Issues and Technical Debt widget to your project dashboard if it's not there already:
Use the differential views to monitor new issues.