Summary

If you have multiple conditional chain command blocks in a row then as soon as the condition is true on the first activation of the chain all of the chain command blocks execute, but when the condition goes back to false, the second command block in line gets executed on more time when you power the chain again, the third even two times more often than expected, the fourth three times, and so on.

Exact bug reproduction

Setup some command blocks exactly like in the attachment picture (Description.png).
All the Chain command blocks are Always Active, the Impulse command block Needs Redstone.

If you power the Impulse command block (by pressing the button), while standing away from the command block with /testfor, nothing should happen.

Now stand close to the block with the /testfor and press the button.

As expected, and working like in 15w35b (and before), all of the chain of command block executed.

Now step back from the /testfor command block and press the button.

The /say 1 command block didn't fire just like one would expected, but surprisingly the other three did.

Continue by pressing the button again.

and again...

and again...

...this last time we finally get no new chat output anymore.

One can clearly see the pattern here, and this works not as intended as well as breaking a lot of use cases for conditional chain command blocks.

How it was in other versions

In the version 15w35b after staying away from the /testfor command block you would immediately get no more feedback in chat from pressing the buttons.

In 15w25c you get close to the /testfor command blocks and press the button twice, getting

after the second click, and then on the next click

and so on, and the decativation would be like in 15w35e.

In 15w35d everything behaves exactly like in 15w35e.