Equipping a constant effect Restore Health/Magicka/Fatigue item will permanently boost the stat it's restoring
Tested on Vanilla (0.35) with Helseth's Ring (player->additem "helseth's ring" 1).
This can be done multiple times. If you equip and unequip Helseth's Ring 10 times in a row, your Health and Fatigue will be permanently boosted by 100 points. This works with Magicka too, though you need mods for get items with Const Rest Magicka. I found the behavior with the following mod: http://mw.modhistory.com/download-37-11658
I'm fairly certain that Attributes and Skills do not suffer from this bug, only Health, Magicka, and Fatigue have this behavior.
(RM-2408 from redmine: created on 2015-02-28 by Tyler Robertson, , closed on 2015-03-02 by scrawl .)