'Object reference not set to instance of an object'
The "Object reference not set to instance of an object" error in ASP.Net has to be one of the most annoying of all error messages giving, as it does, almost no clues as to what the error can be about. And yes, I know it's easy to get more information, but it doesn't really provide any more help than just saying "It broke" would.
It has to be up there with "Object expected" in PHP and the old favourite "Error on line 0 character 0" in JavaScript.