Howdy, pardners! Outlaws of Thunder Junction is here and it’s the rootinest, tootinest, gun-shootinest Magic set we ever did see. It’d be a crime to go out there unarmed (with knowledge!), so we’ve corralled together cards with triggered abilities that might earn players a Warning, as well as some other potentially notable cards. Ready? Saddle up – time to ride!
Three cards have one or more triggers that upgrade to a Warning when missed:

1. Arid Archway
“When this land enters, return a land you control to its owner’s hand. […]”
Maybe this town would be big enough if this land didn’t make you return another one to your hand. While it’s true there’s some small upside to this trigger because it potentially lets you surveil, it’s still generally detrimental. Turns out you need lands to cast spells, and more lands is a lot better than fewer lands. If a player forgets to return another land when they play Arid Archway, they’ll earn a Warning.
2. Calamity, Galloping Inferno
“[Create an attacking token.] Sacrifice that token at the beginning of the next end step. […]”
Say howdy to red’s traditional self-destructing-token maker! As usual, while making another attacking creature is excellent, having to get rid of that creature later is a notable drawback. If a player forgets to sacrifice the token this trigger created, they’ll earn a Warning. The remedy here to undo the zone change never expires, so even if it’s been a couple turns since the delayed trigger was missed, the token will still be forced to get out of Dodge.
“Whenever a player casts a noncreature spell, this creature deals damage to that player equal to the number of noncreature spells they’ve cast this turn.”
Symmetrical triggers like these can upgrade to a Warning if a player forgets to affect themselves. If Aimee controls a Lizard and casts a Lightning Bolt, she needs to remember to deal herself damage or this will upgrade. If Nate casts a Lightning Bolt and Aimee forgets to deal him damage, that doesn’t upgrade!
Three notable cards have a trigger that does NOT upgrade when missed:

“When this creature enters, target player mills two cards.”
Thankfully, this set is free of mandatory self-mill triggered abilities with no upside. Bloodseeker can be self-mill if you point it at yourself, but it isn’t always, which means it isn’t generally detrimental.
“[Exile target nontoken creature]. Return it to the battlefield under its owner’s control at the beginning of the next end step.”
Like most modal spells, Getaway Glamer is pretty flexible in its applications. While players will sometimes target the first mode at their opponent’s creatures, they’ll also sometimes target their own. Maybe Annie needs to temporarily remove her big creature so the Glamer’s second mode can destroy her opponent’s big creature! There are too many possibilities here to solidly pin this as generally detrimental, so this shouldn’t upgrade to a Warning. If the delayed trigger to return something is missed, the window to remedy the error never expires. The exiled creature will be returned immediately when the error is noticed.
“When this creature enters, return up to one target non-Faerie, nonland permanent you control to its owner’s hand. If a permanent was returned this way, put a +1/+1 counter on this creature.”
This trigger is completely optional, and if a player chooses to resolve it they’ll return something that is worse than a 2/2 with flying would be in whatever situation they’re in.
Other notable cards and mechanics:

1. Lazav, Familiar Stranger, Magda, the Hoardmaster, and the whole posse that “triggers only once each turn.”
“[…] This triggers only once each turn.”
Outlaws of Thunder Junction features this template on fifteen different cards. If a player misses one of these triggers, but then tries to remember another instance of the trigger later when the trigger condition is met again, that won’t work!
Let’s take At Knifepoint as an example. Amelia plays Abraded Bluffs and deals 1 damage to Norbert with its enters trigger. She then attacks and deals combat damage. In her second main phase, Amelia casts Burst Lightning targeting Norbert and says “At Knifepoint trigger, make a Mercenary?” Amelia missed the original At Knifepoint trigger that should have happened when she played Abraded Bluffs, but At Knifepoint’s once per turn trigger already happened. She will not get to make a Mercenary the second time she committed a crime this turn.
“Whenever this creature attacks while saddled, put a +1/+1 counter on each other creature you control. You gain 1 life for each of those creatures.”
It’s unlikely a player will entirely miss this trigger; it turns out people like putting counters on their creatures. That said, it’s somewhat more likely that players will forget the “gain life” part of this trigger. If they do, handle it as a Game Rule Violation, not a Missed Trigger.
3. Lassoed by the Law and Mystical Tether
“When this enchantment enters, exile target nonland permanent an opponent controls until this enchantment leaves the battlefield.” and “When this enchantment enters, exile target artifact or creature an opponent controls until this enchantment leaves the battlefield.”
These enchantments don’t return the exiled permanent with a triggered ability. If one of these enchantments is removed from the battlefield and the exiled permanent isn’t returned, we’re in Game Rule Violation territory.
4. Ruthless Lawbringer, Unscrupulous Contractor, and Breeches, the Blastmaker
“[Do something]. When you do, […]” and “[… flip a coin]. When you [win or lose] the flip, […]
Reflexive triggers are sufficiently communicated by taking the preceding action. For instance, if a player resolves Ruthless Lawbringer and sacrifices another creature, but somehow forgets to target a nonland permanent, they’ve committed a Game Rule Violation.
5. Stop Cold
“When this Aura enters, tap enchanted permanent.”
Well, boy howdy, stop the presses, it’s this set’s Thirst! If a player resolves this enchantment but somehow forgets to tap the enchanted permanent, tap the enchanted permanent immediately.
That’s all we’ve got for Outlaws of Thunder Junction, pilgrim. Time for us to mosey off into the sunset with a hopeful Ennio Morricone score. Now you’ve got enough knowledge in your saddlebags to be your players’ huckleberry.
See y’all next time for Bloomburrow. I hear it’s supposed to be a hopping good time.