I stumble over this issue every once in a while: Windows Forms controls will only data-bind when they are visible or have been visible once - this can become an issue when controls are located on a TabPage that has never been shown. The control's bound value then has never been pushed into the UI.
Especially validation-code that checks on properties like TextBox.Text is vulnerable to this - e.g., no matter what the bound value is, TextBox.Text returns an empty string as long as it has not been visible (anyway, in my opinion validation code should always work on model values, not on view properties).
One way to work around this is to check on Control.Created before validating a control's content.
