Hi,
Maybe an outlaying bug : earlier this evening during a fight against a Paladin I began to cast fear, he stunned me (HoJ), but the thing is that despite the debuff+animation, the fear continued its cast and then landed (always when myself stunned). I wasn't lagging or anything, just spamming the only spell in my main action bar.
I think that's pretty clear that while stunned, any action should be cancelled and impossible during the effect.
Sorry Advacity (think that's your nick ?), because of this the fight wasn't fair, but if it could reassure you, the drood ninja'd your HK and honor from me =)
P.S : I recall this bug was present a long time ago, when spamming right before a stun, the cast could still be landed. I think the same thing happened there.