To fix the issue when you have required variants and try to change the tab on the product details page displays the required message do the following:
1. Go to your \desktopmodules\smith.productdetails folder
2. Open the file titled "productdetails.ascx
3. Go to line 23 and add the causesvalidation property to false as highlighted below:
<telerik:RadTabStrip ID="RadTabStripProd" runat="server" Orientation="HorizontalTop" SelectedIndex="0" MultiPageID="RadMultiPageProd" Skin="Simple"
CausesValidation="false" >
We will include this hotfrix in the next cart release. Thanks for reporting the issue!