Only on IE, the textbox postback is triggered when the user scrolls the autocomplete results. I have no problems in Chrome or FF.
<asp:TextBox ID="txtBreakfast" ClientIDMode="Static" CssClass="headerinput" AutoPostBack="true" runat="server" OnTextChanged="txtBreakfast_TextChanged"></asp:TextBox>
<ajaxToolkit:AutoCompleteExtender ID="txtBreakfast_AutoCompleteExtender" runat="server" UseContextKey="true" ContextKey="" Enabled="True" ServicePath="/service/service1.asmx" ServiceMethod="GetFoodNames" MinimumPrefixLength="1" CompletionSetCount="10" TargetControlID="txtBreakfast" CompletionInterval="500" CompletionListCssClass="autocomplete_completionListElement" CompletionListItemCssClass="autocomplete_listItem" CompletionListHighlightedItemCssClass="autocomplete_highlightedListItem" DelimiterCharacters=";,:" ShowOnlyCurrentWordInCompletionListItem="true">
</ajaxToolkit:AutoCompleteExtender>
How it's supposed to work: User starts typing food, then selects the food from the extender div, then a postback occures and it causes additional database information to be displayed based on the food.