Earlier today, I Tweeted about Meandering Towershell, the card that is probably my favorite flavor card in the set. It's... a... slow... turle...
Meandering Towershell is a bad zone-change triggers to miss at Comp REL. Opponent can choose to have it return tapped after combat.— Riki Hayashi (@mtgRikipedia) September 25, 2014
I got a few replies, a lot of them wondering about the details of this Missed Trigger interaction. I did my best to field questions, but Twitter has its limitations. Here then, hopefully, is the bigger picture on this card.
It attacks one turn, then gets lost in the woods only to complete its epic attacking journey on the following turn. This is all fun and games, but it accomplishes this through the use of a very awkward delayed trigger "Return it to the battlefield under your control tapped and attacking at the beginning of the declare attackers step on your next turn." (Trigger condition italicized for emphasis.) As we saw the last few years with Jace, Architect of Thought, remembering triggered abilities across multiple turns is probably one of the hardest memory aspects of the game. This is why so many players use a counter on top of their deck to remind them of upkeep triggers, so that they handle them before they draw a card and nullify the trigger. It's harder to set a reminder like this for a combat trigger, especially on your opponent's turn, which is what made Jace so easy to miss.
Meandering Towershell is possibly a little easier to remember because it is your own combat phase. You could just place the counter on your library anyway to remind you at the beginning of your turn, draw your card, and move straight into combat with Slow Turtle leading the charge.
So what happens when you miss the return trigger? Simply put, you're going to get a tapped Turtle that missed its chance to get in for damage. (As always, this series presumes Competitive REL under the auspices of the IPG.) Zone-change triggers fall under a special class of trigger (often called "Obzedat triggers" because that was the card most popular at the time) that are dealt with thusly:
"If the triggered ability is a delayed triggered ability that changes the zone of an object, resolve it. For these two types of abilities, the opponent chooses whether to resolve the ability the next time a player would get priority or when a player would get priority at the start of the next phase. These abilities do not expire and should be remedied no matter how much time has passed since they should have triggered."
First off, let's clarify when the return ability triggers and when it's missed. It happens "at the beginning of the declare attackers step." This is after you declare the rest of your attackers, which is a turn-based action that happens as the first part of the declare attackers step before either player gains priority (which is when triggers go on the stack). This means that if you let your opponent declare blockers without having returned the Turtle, you've missed the trigger.
Once it has been missed, we deal with it as above, which means that your opponent could choose to have the trigger resolve now, or at the beginning of the next phase. Let's say that you remember as soon as your opponent declares their blocks. They could let the Towershell return now, during declare blockers, which would be mighty generous of them... or they could have it return at the beginning of the post-combat main phase... tapped and attacking, which mostly just means tapped because attacking creatures can't exist outside of combat. Remembering at other times doesn't help you much. On your opponent's turn, you'll just have a tapped Towershell (you can't have an attacking creature during their combat phase). If you remember on a subsequent turn of yours in the pre-combat main phase, your opponent will likely choose to give you a tapped pre-combat Towershell.
Note that unlike other triggers, which often disappear into the aether forever, if this one is discovered later in the game, you always follow the prescribed remedy. "Hey, this Towershell was exiled 10 turns ago!" It still comes back tapped and probably not attacking. As the opponent, you still have no obligation to point out the trigger for your opponent, although you are welcome to do so if you would like them to have a tapped 5/9 for some reason.
One final note for you to sink your rules knowledge into, yes, taking this card with Act of Treason does make it return to you (the Act player) permanently. Fun!