During the Gathering Storm campaign, I noticed that when zombies use their zero round refresh attack 'eat brains' that a single boon causes two stress.

If I understand the rules correctly, this result is triggered even on a miss, which means its very easy for a zombie to get a single boon and cause two stress.

Is this right?

I acutally kind of like it, just checking I'm doing it right.

Yep, i would do the same.

Those boon lines that you cannot use if the action fails are the ones in the line of "Critical Damage", "+x Damage", "+y Critical", etc., or effects that rely/depend on the attack.

I handle it the same way. A zombie trying to eat your brains would be quite stressful regardless of the success or lack thereof.

Any boon, comet, bane or star effect that can be triggered even if the action fails will get triggered as I read the rules. It is a bit strange and I think at one point in the development cycle they had actions recharge even on a miss (the fast quality error gives that away). With recharge on failure it would make a lot more sense.

But according to the rules those stress will be inflicted even on a miss. Players can also get extra maneuvers on a lot of cards even when failing.

But anything that somehow modifies the effect of a success line won't happend if the action fails.

Correct. As Cwell stated pretty well, if it isn't something that modifies or obviously requires a successful completion of the action (such as adding to a damage) then the boon result (and bane results!) can still be triggered. This includes, for example, the universal boon and banes effects. It's one of the strengths of the system, that you can miss/fail but still have some good come out of your action... or vice versa, where your action can succeed but have some negative consequences (due to banes/chaos star, etc).

Great, thanks guys.