Here's a fun one that my player have brought up for the judge hive mind;

Ilharg, the Raze-Boar, as part of it's triggered ability from being declared an attacker, puts a Spark Double onto the battlefield attacking, which then chooses to become copy of a Planeswalker already on the battlefield as it enters the battlefield.

'Whenever Ilharg, the Raze-Boar attacks, you may put a creature card from your hand onto the battlefield tapped and attacking. Return that creature to your hand at the beginning of the next end step.'

'You may have Spark Double enter the battlefield as a copy of a creature or planeswalker you control, except it enters with an additional +1/+1 counter on it if it's a creature, it enters with an additional loyalty counter on it if it's a planeswalker, and it isn't legendary if that permanent is legendary.'

Collectively with my more advanced players we've agreed that the attacking Spark Double would drop from combat but still return to the owner's hand at the beginning of the next end step. However I'm not having any luck finding the specific aspects of the comprehensive rules to ensure that this ruling is completely accurate.

EDIT: this might complicate things;
506.3a If an effect would put a noncreature permanent onto the battlefield attacking or blocking, the permanent does enter the battlefield but it’s never considered to be an attacking or blocking permanent.

Edited William Dwyer (May 14, 2019 07:34:06 PM)

Hi,

Thanks for your questions.

1. Yes, the Double enters the battlefield tapped, but won't be attacking. Since it enters as a planeswalker, not a creature, it won't be attacking at all. It's covered by this rule:

506.3a If an effect would put a noncreature permanent onto the battlefield attacking or blocking, the permanent does enter the battlefield but it’s never considered to be an attacking or blocking permanent.

2. Yes, it will still be returned to your hand at the end of turn. That's covered by this rule:

603.7c A delayed triggered ability that refers to a particular object still affects it even if the object changes characteristics. However, if that object is no longer in the zone it’s expected to be in at the time the delayed triggered ability resolves, the ability won’t affect it. (Note that if that object left that zone and then returned, it’s a new object and thus won’t be affected. See rule 400.7.)
Example: An ability that reads “Exile this creature at the beginning of the next end step” will exile the permanent even if it’s no longer a creature during the next end step. However, it won’t do anything if the permanent left the battlefield before then.

Even though the Double isn't currently a creature, the delayed trigger from Ilharg still knows what permanent its triggered ability put on the battlefield, and the Double will still be returned to your hand, even if it's not a creature.

Nathan Long
Magic Rules NetRep