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Heritage Druid's ability
Tap three untapped Elfs you control: Add GGG to your mana pool.

The bug I found is that (BUG Description):
If you have used Heritage Druid's mana ability once this game,
then whenever you need pay mana (e.g. Casting Spell or Activing Ability), you can get free mana by "activate Heritage Druid's mana ability first and then cancel this ability".
(Notice: You need control at least 3 elfs to be able to activate Heritage Druid's ability)

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For example:
I have one Heritage Druid, two Elvish Warrior (totally 3 Elfs) and 4 Forest on the battlefield (on my side).
I have used Heritage Druid's mana ability last turn.
I have one Pathrazer of Ulamog in my hand.

For casting Pathrazer of Ulamog, I need pay 11 mana which I only have 7. (4 from lands and 3 from Heritage Druid's ability.)
However if I use this Free Mana BUG I could:
Step 1. Choose Pathrazer of Ulamog to cast
------Now Forge ask me to pay 11 mana
Step 2. Activate Heritage Druid's mana ability (no need to choose Elfs to tap)
Step 3. Cancle Heritage Druid's mana ability
------Now Forge only ask me to pay 8 mana.
Seems like cancle Heritage Druid's mana ability also giving mana, also no elf was tapped means I can still active Heritage Druid's mana ability
Step 4. Repeat Step 2~3 once
------Now Forge only ask me to pay 5 mana.
Step 5. Repeat Step 2~3 once more
------Now Forge only ask me to pay 2 mana.
Step 6. Repeat Step 2~3 the lasr time.
------Now Forge sayed Pathrazer of Ulamog is cast, and it is put on the stack.

Basicly this bug means that u can have infinite Green mana.

What if my hand is not a Pathrazer of Ulamog, but a Emrakul, the Aeons Torn?
emmm

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I have made a puzzle for testing this bug.
Unzip it and place in the folder ../[your Forge folder]/res/puzzle/
Then open Forge, in the Puzzle Mode, choose and start Bug Test - Heritage Druid's Free Mana BUG.

The Free Mana BUG is not a always-true bug.
Means that to reproduce this bug you needs to SUCCEFULLY use Heritage Druid's ability once first. (I call this action as "turn on trigger of this bug")

Also the trigger is not global. So for each Heritage Druid, you need to trigger each one individually to let each one be able to use the bug. (and if a bug triggerd Heritage Druid left the battlefield and then return back, it needs to trigger again to use the bug)

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In case someone may be unable to reproduce the bug, I have made a GIF below:
https://gfycat.com/gifs/detail/NextCorruptDormouse

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ps. Bug found on Forge Release 09/29/2017 ver 1.6.4

Sorry for the confused BUG description, that's because I am not a English nativespeaker.