since upgrading to jQuery v1.9, and using the latest jquery.validate.unobtrusive (Microsoft.jQuery.Unobtrusive.Validation" version="2.0.20710.0")
my code crashes with
Error: Invalid character
The reason seems to be that jquery.validate.unobtrusive is calling jQuerys parseJSON with an undefined value. This was accepted in earlier version of jQuery, but v1.9+ does correctly now throw an error.