Failure detection in Web Applications is a topic of paramount importance. There are several papers in the literature about end-to-end monitoring of web-sites. Most of those systems use a distributed set of probes, located in different places in the Internet, and conduct some remote checking of the main HTML pages of the web-sites. This approach seems viable and many web-sites are already using some commercial services that provide this type of monitoring. In this paper we present new insights about the nature of user-visible failures in web-sites. We conducted an experimental study in the Internet by monitoring a set of 20 well-known web-sites during a one-month period. The results achieved are interesting for the whole community that is studying the reliability of the Internet and Internet Sites.