Some Wall Street Banker must have put a hex on my code

This is really strange:

I have a button on my page which has a Click() event:

    protected void btnRun_Click( object sender, EventArgs e )
    {
      lbl_St8Hint.Visible = (DropDownList_Criteria.SelectedIndex == ST8_INDEX);
      if (DropDownList_Criteria.SelectedIndex == ZIP_INDEX)
      {
        FindForZip();
      }
      else if (DropDownList_Criteria.SelectedIndex == CITY_INDEX)
      . . .

The above event IS hooked up to the button.

I have a breakpoint on the first line (I tried it on the second line, too, with the first line commented out).

I run the site with debugging (F5)

I hit the button.

My code is not reached. The breakpoint doesn't stop the ...