Issue with WPF validation(IDataErrorInfo) and tab focusing

Tab items tend to mess up with adorners (although I don't know why, I experienced it).

I could reproduce your problem.

Solve it by wrapping the contents of the TabItem with an AdornerDecorator.

So:

<TabControl >
    <TabItem Header="tabItem1" Name="tabItem1" GotFocus="tabItem1_GotFocus">

        <AdornerDecorator>

           <Grid>
           ....
           </Grid>

        </AdornerDecorator>

    </TabItem>
    ...
</TabControl>