Aimed Shot bar shows a 'Failed' message once the cast completes
(Note: The shot does fire as intended, just a confusing message / visual indication)

I can confirm this bug as well. Multishot is not affected.

Found the problem, when casting aimed shot, the countdown actually goes to around -0.3 seconds (for me at least), so if you change the "0.1" in this line to something greater than that you'll not see the failed message.

if addon_data.hunter.cast_timer > addon_data.hunter.cast_time + 0.1 then

This however is just a quick and dirty fix, it doesn't solve the issue where aimed shot countdown goes negative, which is the root cause of this, don't have time right now to look but will look at it some time after thanksgiving.

The root cause of this is that Aimed Shot has an actual cast time of 3.5 seconds (3 seconds for the cast, 0.5s delay which is added to ALL non-instant cast ranged attacks), and the attack speed bonus from your quiver reduces the overall 3.5s cast time. Check out my bug report for more information