What is the “checkerboard” issue and why does it matter? Thank you.

Just what does it imply when we say that there has been a "data leak?"

As of the time of writing, Damon Davis was completing out the paperwork for Drano Plumbing. He estimated a $50,000 net profit would be made. He counted the columns of the balance sheet and knew how much was in the credit column ($300,000) and how much was in the negative column ($400,000). What, in your opinion, was the most plausible reason for the discrepancy in the results? What should he do next to identify the issue's cause if this wasn't the cause of the issue?

Please clarify what you mean by "header-based vulnerabilities." Name three instances that come to mind.