Sitecore is not defined in webedit

After a migration to sitecore 6.5, the webedit was always say with a message "loading" and we saw an error message "Sitecore is not defined" a javascript.

To make it short the resolution is really easy: you just need to remove the following code if you had it before the migration

<sc:Placeholder ID="webedit" Key="webedit" runat="server" />

If you want to go a bit deeper into the cause of this error, the javascipt who cause this error is automatically regenerate by sitecore when not exist. If you need to control which codes are inserted into the page when you are in webedit mode you may overide the following configuration. This one have evolve into the version 6.5 and this is the cause of our error.

<pageextender type="Sitecore.Layouts.PageExtenders.WebEditPageExtender" />