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Describe the Bug:
1- Oath of Chandra 's ability is optional.
2- I cast Oath of Chandra selected to use ability due to bug 1,AI had only Abbey Gargoyles with protection from red and game stack as I was prompt to target opponent's creature and that couldn't be targeted.

*Can somebody please explain to me why is "At the beginning of each end step..." worded like this instead of the "At the beginning of the end step...". That "each" either is redundant as I understand it or makes me think that Oath of Chandra should trigger on both players end step "if a planeswalker entered the battlefield under your control this turn".
Which card did behave improperly?
Oath of Chandra

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
Manalink 2016/08/27: Eldritch Moon v2, duel

What exactly should be the correct behavior/interaction?
1- Oath of Chandra 's ability is mandatory.
2- Oath of Chandra 's 1st ability doesn't stack game if opponent's creature can't be targeted.
Are any other cards possibly affected by this bug?
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It's both - redundant and should trigger on both players' end steps if a planeswalker entered the bf under your control that turn. You can't cast one normally, but there's plenty of ways to cheat it into play.

I think the reason they usually include "each" in the templating for end and upkeep steps is to prevent the cards from being misread as either "your end/upkeep step" or as "the first end/upkeep step after this entered the bf".

What is the difference between "the first end/upkeep step after this entered the bf" and "At the beginning of each end step...". Give an example please.

Nothing's actually templated as "at the beginning of the first end step after this enters the battlefield" - something that did that would instead be "When this enters the battlefield, [do something]... [same paragraph] At the beginning of the next end step, [do something else]." It runs the delayed trigger exactly once, at the start of the next end step after its primary trigger resolves. A couple examples are Flickerwisp and Swift Warkite.

"At the beginning of each end step" happens at the end of every turn. Plenty of cards use this wording; Wound Reflection is typical. It's functionally identical to "At the beginning of the end step", which plenty of cards also use. Ball Lightning is typical. The latter ones tend to be ones that say "bounce this card" or "sacrifice this card", so the word "each" gets omitted because it implies it's likely to happen more than once. If you Stifle the end-of-turn-trigger, though, it does happen more than once - at the end of every turn, as before.

This has been fixed in dev 778ccb5 version and now is closed.