“Warning: Suspicious Path Detected in Client Request!”

A potentially harmful Request.Path value was detected from the client (?).

Description: An unhandled exception has occurred during the processing of the current web request. Please check the stack trace for further details about the error and its source in the code.

Exception Details: System.Web.HttpException: A potentially harmful Request.Path value was detected from the client (?).

Source Error:

Stack Trace:

[HttpException (0x80004005): A potentially harmful Request.Path value was detected from the client (?).]
   System.Web.HttpRequest.ValidateInputIfRequiredByConfig() +570
   System.Web.PipelineStepManager.ValidateHelper(HttpContext context) +54