You haven't defined what you mean by "single occurrence"? To me being low one month, or during a two months project can be a single occurrence, but that wouldn't lead to a "bad perf review" (at least not in my team). Being clearly underperforming for a complete year without changes after setting up intermediate goals for improvements is not a "single occurrence". When a Sr. Staff which is less productive than other junior engineers, and requires the same level of guidance, something isn't right.