A URL has & character there, for example:
http://mywebsite.com/t123/This_&_That
ASP.NET generate the error "A potentially dangerous Request.Path value was detected from the client (&)".
A potentially dangerous Request.Path value was detected from the client (&).
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Web.HttpException: A potentially dangerous Request.Path value was detected from the client (&).
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace:
[HttpException (0x80004005): A potentially dangerous Request.Path value was detected from the client (&).]
System.Web.HttpRequest.ValidateInputIfRequiredByConfig() +12353567
System.Web.PipelineStepManager.ValidateHelper(HttpContext context) +166