An example, from the AD&D 2e DMG:
For example, Beornhelm the Ranger carelessly dons a helm of alignment change and switches to chaotic evil alignment--something he didn't want to do! Exerting its influence over him, the helm compels Beornhelm to commit all manner of destructive acts. Although unable to resist, Beornhelm keeps looking for an opportunity to escape the accursed helm. Finally, after several misadventures, he cleverly manages to trick an evil mage into removing the helm, at which point he is restored to his previous alignment.
The Helm of Opposite Alignment (no helm of alignment change item is listed in the DMG) is described as instantly changing the wearer's alignment to one in direct opposition; LG to CE, NN to any of the four extremes, etc. This alteration is explicitly stated to be mental and desired by the wearer. Further, it is permanent to the extent that it can be undone only by a Wish.
The Helm's description seems to contradict the example given in the above-quoted "Effects of Changing Alignment" section. Which is accurate? Are both true by virtue of the existence of a Helm of Alignment Change, different from the Helm of Opposite Alignment, that I haven't been able to find?