Are you referring to old power zooms like the one's for the F3AF and the Olympus Push-button zoom AF SLR lenses? Most of them weren't very good optically. Or something more like a Fujinon Video zoom lens?
Some of the PZ zooms aren't really parfocal. As you zoom, the camera adjusts the focus by a known amount to compensate. Even so, they recommend you critically focus zoomed in and then zoom out to the desired focal length.