Recently hit a WPF issue with IDataErrorInfo where using ValidatesOnDataErrors with a TextBox creates a border around the textbox that doesn’t go away when used with an Expander. Screen shot below before closing expander: 

Screenshot after closing expander:


The workaround?  Create a style for Textbox (or TextboxBase, depending) which blows away the validation error template like this:

<Setter Property="Validation.ErrorTemplate" Value="{x:Null}"/>

And then manually sets the border, like this:

<Setter Property="ToolTip"  
Value="{Binding RelativeSource={x:Static RelativeSource.Self},  
<Setter TargetName="Border" Property="Background" Value="Red"/>  

That did the trick. Download a complete sample of the fix.