private void Form1_Load(object sender, EventArgs e)
{
    maskedTextBox1.Mask = "00/00/0000";

    maskedTextBox1.MaskInputRejected += new MaskInputRejectedEventHandler(maskedTextBox1_MaskInputRejected);
    maskedTextBox1.KeyDown += new KeyEventHandler(maskedTextBox1_KeyDown);
}

void maskedTextBox1_MaskInputRejected(object sender, MaskInputRejectedEventArgs e)
{
    if (maskedTextBox1.MaskFull)
    {
        toolTip1.ToolTipTitle = "Input Rejected - Too Much Data";
        toolTip1.Show("You cannot enter any more data into the date field. Delete some characters in order to insert more data.", maskedTextBox1, 0, -20, 5000);
    }
    else if (e.Position == maskedTextBox1.Mask.Length)
    {
        toolTip1.ToolTipTitle = "Input Rejected - End of Field";
        toolTip1.Show("You cannot add extra characters to the end of this date field.", maskedTextBox1, 0, -20, 5000);
    }
    else
    {
        toolTip1.ToolTipTitle = "Input Rejected";
        toolTip1.Show("You can only add numeric characters (0-9) into this date field.", maskedTextBox1, 0, -20, 5000);
    }
}

void maskedTextBox1_KeyDown(object sender, KeyEventArgs e)
{
    // The balloon tip is visible for five seconds; if the user types any data before it disappears, collapse it ourselves.
    toolTip1.Hide(maskedTextBox1);
}