I have following simple code:
<%@ Page Language=\"C#\" AutoEventWireup=\"true\" CodeBehind=\"testForm.aspx.cs\" Inherits=\"Orbs.testForm\" %>
In my case, this happened because I had referred to that dropdown using a local dropdownlist variable, and disposed off that variable at the end of a function. I was adding the first item to the dropdownlist after setting a datasource. Took me 2 hours to find this, and worked perfectly after I removed the dispose call.