Deus ex Machina
The idea of checking websites for bad reputation is so popular that there are tons of such services offered. There is even UrlVoid a website that uses a number of services to scan websites for exploits, malware and bad business practices. UrlVoid is very good but it misses out some of the big players like SiteAdvisor.
That's where we can help. Using crack tracker's screen scraping engine, we wrote a free desktop tool that checks a website against all major "virus scanners". It is very simple to use, just type the website URL and click on Get reputation button. It then queries all the website scanners and reports the results in a list. You should look out for warning icons which indicate trouble with the scanned website. You can double click on a result to see the original website report details.
Download WebRep, the free web reputation checker v1.4 (150 KB)
Update 21-Nov-17: This tool is no longer updated and is discontinued. To check your website against lots of content monitors try virustotal URL scanner
This is not a hypothetical scenario, it does happen. If your website exhibits a sudden and mysterious drop in sales it could be down to a false positive threat assessment. So you need a tool like WebRep to check your website's reputation from time to time. If you happen to get a negative rating you need to act quickly and try to restore a clean verdict. This is easier said than done as each scanning service has its own method for reporting false positives (e.g. here's what you have to do for site advisor). No matter what though, if you are in business on the internet, it is imperative you maintain a clean record, so you must bury your anger and go through whatever hoops and loops they force you to. It's hard life being a webmaster but at least now you have an extra helping tool in your hands!