With Sitefinity, users are able to change the web address (a.k.a. url) for virtually every content type- blog post, page, event, etc.  This setting is found under URL (see screenshots below for examples).

Because of this flexibility, there are times when an error may be generated on the front end because a URL either has a special character in it, or because there are two pieces of content with the same URL.  When a piece of content is created, Sitefinity does check for existing links to prevent errors. More often problems come up when a special character is automatically generated in the URL. This happens when an administrator creates a piece of content with special characters in the title; the most frequent offender tends to be an exclamation mark (!).  

It is very important that you do not ever create an event, page, blog article with special characters in the title. Characters that cause problems include the percent sign (%), star (*), parenthesis (), exclamation mark (!), quotes (" or '), pound or hash (#), etc.  

If you find a piece of content with one of these special characters in the title, you should check that the link does not include that character. You can do this by going into the content settings and reviewing the link displayed directly under URL. If a special character is listed there, use the 'Change' button to remove the character and republish that content piece.   

For pages, this setting can be found here:

For events and blogs, this setting can be found under 'More options':