Recently I came across an error in an asp.net page that took a fair amount of troubleshooting to resolve.  I had an asp.net page that was using a master page and within the aspx page, I had a grid view.  On the aspx page, I also had a drop down list that posted back to the server when the selected index changed.  On post back, I received the below error: 

An error has occurred because a control with id 'ctl00$ContentPlaceHolder1$gvFiles$ctl05$ctl00' could not be located or a different control is assigned to the same ID after postback. If the ID is not assigned, explicitly set the ID property of controls that raise postback events to avoid this error.

System.Web.UI.Page.ProcessPostData

After some digging, I determined the error was due to multiple asp:ButtonField controls being placed in the GridView.  Since you can not assign an ID to these controls the above error was being raised.  I did not find out exactly why, but at a higher level, the issue was due to my master page having view state disabled.  Once I changed this so my master page had ViewStateEnabled="true" (I had previously set it to false) the error went away.  If you have viewstate turned off at the page or gridview level you may also encounter this issue.