Invalid Id: An unexpected error has occurred. Your development organization has been notified

This obscure Salesforce error appears from time to time when fetching a custom url parameter in an extension controller class.

Id recordId;
recordId = ApexPages.currentPage().getParameters().get('rid');
if(rid!=null&&rid!=''){ //ERROR REPORTED ON THIS LINE

Resolution is to change the data type to String. There seem to be more implicit conversions from String to Id than the other way.