Here’s an odd one.
We had customized our master pages by adding in the Search WebPart, so we could customize some of the parameters
We changed the following:
<asp:ContentPlaceHolder id="PlaceHolderSearchArea" runat="server"></asp:ContentPlaceHolder>
To the following:
<asp:ContentPlaceHolder id="PlaceHolderSearchArea" runat="server">
<SPSWC:SearchBoxEx webpart=”true” runat=”server” __webpartid=”{2883947B-2B5C-4A56-92B3-33B7383F7C0D}” id=”SearchBoxEx1″><WebPart xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=”http://www.w3.org/2001/XMLSchema” xmlns=”http://schemas.microsoft.com/WebPart/v2“>
<Title />
<FrameType>None</FrameType>
<Description>Used to search document and items.</Description>
<IsIncluded>true</IsIncluded>
<ZoneID>ImportedPartZone</ZoneID>
<PartOrder>0</PartOrder>
<FrameState>Normal</FrameState>
<Height />
<Width [...]
Read the complete post at http://statto1974.wordpress.com/2007/10/29/be-careful-when-editing-the-applicationmaster-page/