IMPORTANT NOTE: This problem was found when I was working with AJAX RC1.
Recently, I tried to add an AutoCompleteExtender for each item in a repeater (which itself is inside an UpdatePanel). In each item, I added also Edit/Update links, textbox and AutoCompleteExtender. Here is the HTML view
However, it seems that TargetControlID property is not affected when the autocomplete set inside the repeater/updatepanel (Actually, don't know which one is the reason). But what I did is to set the TaregtControlID property of the extender in runtime, when the user trying to edit a certain item.
That is all!