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References:

• "getElementById: Returns the Element whose id is given by elementId. If no such element exists, returns null. Behavior is not defined if more than one element has this id. Parameters: elementId of type DOMString
  The unique id value for an element."
  DOM 1 HTML (2nd edition), getElementById
• "getElementsByName: Returns the (possibly empty) collection of elements whose name value is given by elementName. Parameters: elementName of type DOMString
  The name attribute value for an element."
  DOM 1 HTML (2nd edition), getElementsByName
• "getElementById: Returns the Element whose ID is given by elementId. If no such element exists, returns null."
  DOM 2 Core getElementById http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-getElBId and DOM 3 Core getElementById http://www.w3.org/TR/DOM-Level-3-Core/core.html#ID-getElBId
• getElementsByName: With [HTML 4.01] documents, this method returns the (possibly empty) collection of elements whose name value is given by elementName.
  DOM 2 HTML getElementsByName http://www.w3.org/TR/DOM-Level-2-HTML/html.html#ID-71555259

This bug has been reported at connect's IE beta feedback as bug 333979 and has also been reported at Web Bug Track bug 152: getElementById returns incorrect objects in IE.