C#

int lastValue = 0;
private void numericUpDown1_ValueChanged(object sender, EventArgs e)
{
    if (this.numericUpDown1.Value < 10)
    {
        this.numericUpDown1.Increment = 1;
    }
    else
    {
        this.numericUpDown1.Increment = 10;
    }

    if ((this.numericUpDown1.Value == 10) && (this.lastValue > 10))
    {
        this.numericUpDown1.Increment = 1;
    }
    this.lastValue = (int)this.numericUpDown1.Value;
}

if ((numericUpDown1.Value == 10) && (lastValue > 10))
			{
				numericUpDown1.Increment = 1;
			}
			lastValue = (int)numericUpDown1.Value;

int lastValue = 0;
private void numericUpDown1_ValueChanged(object sender, EventArgs e)
{
    if (this.numericUpDown1.Value < 10)
    {
        this.numericUpDown1.Increment = 1;
    }
    else
    {
        this.numericUpDown1.Increment = 10;

        if ((this.numericUpDown1.Value == 10) && (this.lastValue > 10))
        {
            this.numericUpDown1.Increment = 1;
        }
        else if (this.numericUpDown1.Value == 11)
        {
            this.numericUpDown1.Value = 20;
        }
    }
    this.lastValue = (int)this.numericUpDown1.Value;
}

private void nud_ValueChanged(object sender, EventArgs e) {
	decimal newValue=nud.Value;
	if (newValue>10) {
		bool incrementing=nud.Tag==null?true:nud.Value>(decimal)nud.Tag;
		newValue=((int)newValue/10)*10;	// round down to multiple of 10
		if (incrementing) newValue+=10;
	}
	nud.Tag=newValue;
	nud.Value=newValue;  // fires on change only!
}