while playing an amazon using fire elementals i made the lvl 15 runeword lumen arcana in my chest armor rendering it "unwearable" though still equipped as my characters level was not 15+. the bug i noticed is the +1 to all skills from the runeword still seemed to take effect. i was able cast an additional fire elemental even though the skill tooltip was shown correctly. unequipping the armor made one of my fire elemental's disappear which confirmed my suspicion.
though it sounds rather insignificant, this bug "could" mean that it applies to any number of +x to all skills for any skill...for example a level one character fully equipped with 1 socket items then a high lvl runeword made in each. granting a large number +x to all skills. this is why i posted this bug report.
if this bug only happens with lumen arcana or maybe fire elementals then i suppose such a bug could go unnoticed, if not it could be a rather huge exploit.
EDIT: this was in single player. havnt tested recreating bug online or with any other skills/runewords