This AA reads as such: This ability reduces your chance of failing on apoison combination by10, 25, and 50 percent. It also reduces the time to apply poison by 2.5 seconds per ability level. Once one point is applied to this ability, you will never again fail in poison application
bolded the section of the description that is especially of note.
this is flat out untrue at this time, i am currently 200 in apply poison and have 1 point in poison mastery and i still not only see fails but they are seemingly just as often as before putting the point in to poison mastery. Screenshots of this happening can be provided upon request however this bug is quite easy to replicate should one want to try and test it.