Operation is not valid due to the current state of the object.

I was trying to retrieve more than 1000 items from List and save back with some modification, then I faced this issue :Operation is not valid due to the current state of the object.

I first try with resource throttling configuration but found no any change in the error.
After surfing, I found this Forum
And after adding following parameter in AppSettings in Web.config
<add key="aspnet:MaxHttpCollectionKeys" value="20000" />
The issue has been resolved.

Now I want to know that what it does and why we use this for bigger data (While Retrieving and saving/updating back)

Then I found the following link and msdn article which tells us following:

aspnet:MaxHttpCollectionKeys:-
Limits the maximum number of items that can be present in any of the client-provided dictionaries of the HttpRequest object.
Default Value is 1000 and
<appSettings>
      <add key="aspnet:MaxHttpCollectionKeys" value="1000" />
    </appSettings>
Now, you can change the value as I shown above.If you explore it a bit more then:

One of the things this update does is update the System.Web.dll. 
In the new System.Web.dll they modified some of the internal 
classes and put a limit on the number of form input elements that 
can be submitted to a page.  This was most likely done to thwart
 against the hash DOS attack.  The default number of input html elements
 that can be submitted is now 1000.

Updated :- 01/30/2014

When trying to run the timerjob using console application it is throwing same error, though I have already set the above setting in appsettings also used the RunWithElevatedPrivilaedges.

After surfing net, I found the solution that during console application we need to set
HttpContext.Current = null;