Why can't I log in to the Sitecore website with my Salesforce credentials?

This is when the Security Connector is used to store the Sitecore login credentials in the Salesforce contact or lead record. 

Your login control needs to fully qualify the username to login to your Sitecore website. For example, the login name “JackRogers” becomes “salesforce\JackRogers”. For consistency of use, it is recommended the Sitecore login control be modified to automatically append “salesforce” to the front of the username.

Also be aware that S4S caches the membership data from Salesforce. S4S will read the cache data until it expires then refresh the cache from Salesforce on the next query. Any time a password is changed in Salesforce using S4S, it is a good idea to refresh the ASP.NET cache so the new values will appear in Sitecore.