If you are working with a SharePoint master page and trying to get away from using the multiple tables that the default master page uses, the edit consoles (WSSDesignConsole and PublishingConsole) may create problems for you.
The issue: The WSSDesignConsole and the PublishingConsole are user controls, which means that the interface code for the consoles is stored outside of the master page and is locked down. The interface code for these consoles starts off with a table row tag (<tr>) and not a table tag (<table>), so the potential end result is your master page rendering differently than what you have specified in the master page code. The consoles may cause your table cells or rows to end prematurely, thus throwing off your design.
The fix: Inside of the content placeholder, add wrapping table tags (<table>) around the user control. Here is an example:
<!--- --- --- Edit Consoles --- --- --->
<!-- Edit console that appears in page Edit mode -->
<asp:ContentPlaceHolder ID="WSSDesignConsole" runat="server">
<table><wssuc:DesignModeConsole id="IdDesignModeConsole" runat="server"/></table>
<!-- Publishing console control -->
<asp:ContentPlaceHolder ID="SPNavigation" runat="server">
<table><SharePoint:DelegateControl runat="server" ControlId="PublishingConsole"
PrefixHtml="<tr><td colspan="4" id="mpdmconsole" class="ms-consolemptablerow">"
<!--- --- --- End of Edit Consoles --- --- --->