By default, simple binding updates a bound data source property value as part of Control validation.  Control validation occurs when a Control loses focus which occurs when a user Tabs or Clicks off a Control (see Controlling the Binding Operation for more information).  If an error occurs as part of the validation process, Windows Forms will cancel the validation and focus will not move to the next Control.  For example, assume you bind a TextBox "Text" property to an integer property on a data source.  When s user Tabs off the TextBox, validation occurs and the data binding engine (Binding type) will attempt to update the data source property with the bound Control value (TextBox.Text).  If an error (exception) occurs as part of the data source update, the default behavior of the binding engine is to fail validation which prevents focus from leaving the TextBox. 

It is important to note that if an error occurs then the binding engine must assume that the value displayed to the user (TextBox.Text) is inconsistent with the value in the data source property.  In this scenario it is important to neither make it appear as if the binding was successful (silently fail) or display an intrusive dialog.  For this reason, the default behavior is to "trap" the user on the Control until they enter a successful value.

Sample: Using an ErrorProvider and ToolTip to show error status (VS 2005) (VS Project: SimpleBindingTabIssue)

In VS 2005 there are two ways you can provide feedback to a user when a binding error occurs.  The easiest way is to add an ErrorProvider to the Form and set ErrorProvider.DataSource to the same data source used by the Binding.  In addition, you can also add a handler to the BindingComplete event for the Binding and write your own customer error handling logic.  In the sample below, the custom logic shows a ToolTip:

/*******************************************************************
* Setup (using the Visual Studio Form designer):
* 
* Add 1 TextBox to the Form (textBox1)
* Add 2 CheckBoxes to the Form (showErrorToolTipCB
* and showErrorProviderCB)
* 
* Add the following public property to the Form class:
*   
*   private int _intValue;
*
*   public int IntegerValue
*   {
*       get { return _intValue; }
*       set { _intValue = value; }
*   }
* 
* Add the following code to the Form.Load event:
******************************************************************/

ToolTip         errorTT = new ToolTip();
ErrorProvider   ep = new ErrorProvider(this);
Binding         tbBinding = new Binding("Text", this, "IntegerValue", true);

/* Add binding */
this.textBox1.DataBindings.Add(tbBinding);

    /* Hook BindingComplete to allow navigation */
    tbBinding.BindingComplete += delegate(object bind, BindingCompleteEventArgs args)
    {
    if ((this.showErrorToolTipCB.Checked) && !String.IsNullOrEmpty(args.ErrorText))
    {
  /* Show ToolTip */
  errorTT.Show(args.ErrorText, this.textBox1, 2000);
  }
    };

    this.showErrorProviderCB.CheckedChanged += delegate
    {
    ep.DataSource = (this.showErrorProviderCB.Checked ? this : null);
    };

    /* Initialize ToolTip */
    errorTT.IsBalloon = true;
    errorTT.ToolTipTitle = "Binding Error:";

    /* Initialize ErrorProvider */
    ep.BlinkStyle = ErrorBlinkStyle.NeverBlink;
}

Sample: Allowing a user to Tab off a Control when a binding error occurs (VS 2005) (VS Project: SimpleBindingTabIssue)

Control validation occurs when a Control loses focus (see Controlling the Binding Operation for more information).  When the user Tabs off the TextBox, validation occurs and the data binding engine (Binding type) will attempt and update the data source property with the bound Control value (TextBox.Text).   If an exception occurs, validation will fail and the focus will not leave the bound Control.  To allow focus to leave, you can add a BindingComplete event handler and set e.Cancel to false:

/*******************************************************************
* Setup (using the Visual Studio Form designer):
* 
* Add 1 TextBox to the Form (textBox1)
* 
* Add the following public property to the Form class:
*   
*   private int _intValue;
*
*   public int IntegerValue
*   {
*       get { return _intValue; }
*       set { _intValue = value; }
*   }
* 
* Add the following code to the Form.Load event:
******************************************************************/

Binding         tbBinding = new Binding("Text", this, "IntegerValue", true);

/* Add binding */
this.textBox1.DataBindings.Add(tbBinding);

    /* Hook BindingComplete to allow navigation */
    tbBinding.BindingComplete += delegate(object bind, BindingCompleteEventArgs args)
    {
       /* Allow user to Tab off */
      args.Cancel = false;
    };
}