Had a bit of trouble tracking this down, but finally narrowed it down to these 2 controls being on the same page: Menu and ToolTip.
Create a menu:
<code>
<asp:Menu ID="menu1" runat="server" BackColor="#B5C7DE"
DynamicHorizontalOffset="2" Font-Names="Verdana" Font-Size="0.8em"
ForeColor="#284E98" StaticSubMenuIndent="10px">
<StaticSelectedStyle BackColor="#507CD1" />
<StaticMenuItemStyle HorizontalPadding="5px" VerticalPadding="2px" />
<DynamicHoverStyle BackColor="#284E98" ForeColor="White" />
<DynamicMenuStyle BackColor="#B5C7DE" />
<DynamicSelectedStyle BackColor="#507CD1" />
<DynamicMenuItemStyle HorizontalPadding="5px" VerticalPadding="2px" />
<StaticHoverStyle BackColor="#284E98" ForeColor="White" />
<Items>
<asp:MenuItem Text="Test1"><asp:MenuItem Text="Sub1"></asp:MenuItem></asp:MenuItem>
<asp:MenuItem Text="Text2"></asp:MenuItem>
</Items>
</asp:Menu>
</code>
Then create a tooltip:
<code>
<aspnettooltip:tooltip ID="Tooltip1" runat="server">
<TargetContentTemplate>
<img alt="" src="Images/box.gif" />
</TargetContentTemplate>
<ToolTipContentTemplate>
Test
</ToolTipContentTemplate>
</aspnettooltip:tooltip>
</code>
When you run the page and move the mouse over the menu items, the menu item flashes between the styles tied to the Hover state and the non-hovered state. This seems to be only with IE7, it works fine in FireFox 3.5