At What Point Does a Negative Search Result Become a Real Business Problem?

I've been thinking about how businesses evaluate reputation risks online.

Not every negative review, article, or forum post causes measurable damage, yet some seem to have a significant impact on trust, lead generation, and customer acquisition.

For those with experience in SEO, marketing, or reputation management:

How do you determine whether a negative search result is actually hurting a business?
Are there specific warning signs to watch for?
Have you ever seen a single search result affect conversions or sales?
What metrics do you use to measure reputation impact?

I'm interested in hearing how others assess the severity of online reputation issues and decide when action is necessary.