It sometimes happens that commandblock are being activated in the wrong order.
This happens randomly witheout any obvious cause or structure - but it depends on the position/coordinates
To illustrade this effect it's best to stack two commandblocks on top of each other.
The lower commandblock has the command "say 1"
The top commandblock has the command "say 2"
Then you put a redstone on the top and power that redstone, so both commandblocks will be activated.
What should happen:
The top commandblock should be powered slightly before the commandblock under it.
Thus first the upper one should say "2" and after that the lower one outputs "1" in the chat.
What actually happens:
Well, most times thats actually the case and everything works as expected.
In the chat you will be able to read "2" followed by "1"
But occasionally the lower commandblock will be powerd first, thus it says first "1" and then "2".
In the screenshots you are able to see it.
Most times the upper commandblock is powered first, then the lower one.
In the chat is says "2 1"
But on some places the lower commandblock ist powered first, so it says "1 2". This will happen everytime and you can repeat it over and over.
It depends on the coordinates where the commandblock stack is placed.
To replicate this bug, just randomly set up these stacks of commandblocks with the redstone on top. On some positions this symptom will happen.
This bug can be reproduced from 1.7.x - 1.8
Please do not mark unreleased versions (anything starting with "Future Version") as affected. You don't have access to them yet.