VeriSign's sitefinder, which has angered the web-surfers (feed a misspelled or non-existent URL to your browser and you will know what I am talking about) and others, and for this reason, has been target of numerous lawsuits.
However, the problems faced by web-surfers appears to be a minor irritant compared to the grave problems being faced by applications that fetch information through URLs and Web Services, which rely on URLs for addressing. A lot of these have been coded on the assumption that a mis-spelled or non-existent host will be recognized by the returned error code but are now getting HTML documents, which they do not understand or expect. Steve has documented his travails with Apache Axis due to sitefinder.
Looks like we will all have to learn new ways of handling error ;-(