Clearly there are a lot, a lot of strategies out there that are vulnerable to Mistake. Searching is absolutely essential to consistency in many cases. Fire Fists, Constellar, Madolche, Harpies, and Evilswarm need to fetch cards on a frequent basis. From Fire Formation Tenki activations to Constellar Sheratan Summons, the list of effects that Mistakes puts a stop to is unbelievably long. Neither Madolche Ticket nor Hysteric Sign will trigger, and Evilswarm Ophion can't add Infestation Pandemic to protect itself while Mistake's active. Even strategies that use minimal searching can be hurt, although this fact is made irrelevant if you contextualize Mistake as a Side Deck card only.
So what about decks that make searching into something that's necessary rather than just desirable? Dragon Rulers are particularly known for their ability to search eachother out. Each Ruler adds a Dragon to the hand when it's banished, and Tempest, Dragon Ruler of Storms can add any Dragon with its effect. There's a full twelve cards there that lose a part of their functionality due to Mistake. What's more is that those effects are necessary to keep Rulers form outright running out of cards. It's their means of generating card advantage, and it's necessary to keep resolving those effects. Otherwise Summoning a Dragon puts too great a strain on your resources. Making an Xyz Summon with Mistake out would cost you too many cards, and the worst is yet to come: when Mistake's active, you can't use Tempest to try and grab a Blaster to destroy it. Mecha Phantom Beast Dracossack's a possible out, but Summoning it will eat up a huge amount of resources.
With Imperial Iron Wall and Debunk already popular sides against Dragons, what does Mistake have to offer? The first benefit that it provides is to decks that can't play Iron Wall. Debunk's certainly effective, but it doesn't carry the same stopping power as Iron Wall. For decks like Agents and Lancer Frogs, Mistake's a much stronger alternative. The second benefit, and probably the more important of the two, is that Mistake performs better in the Dragunity Ruler match-up. Iron Wall doesn't stop Dragunity Dux from making Scrap Dragon, but Mistake can turn off Dragon Ravine and prevent your opponent from setting up those plays. While Ravine can still be used to send cards to the graveyard, it's another case where you'll begin to run your opponent out of resources very quickly using this trap.
Stopping the occasional search is nice, but we're barely scratching the surface of what Mistake's truly capable of. Like Macro Cosmos, Imperial Iron Wall, or Gozen Match, Mistake has the power to completely shut down certain strategies...especially those that are totally dependent on searching cards. One such deck is Spellbooks, and Mistake's absurdly good at disrupting their plays. Spellbook Magician of Prophecy and Spellbook of Secrets are crucial to getting the ball rolling and making plays. Many of the Spellbooks in the deck require an on-field Spellcaster. If your opponent can't search for Spellbook Magician of Prophecy with Secrets, their entire early game goes out the window. Additionally, Spellbook Library of the Crescent can't be activated while Mistake's in play.
Again, the comparisons to Imperial Iron Wall are unavoidable. Wall's great at stopping Spellbook of the Master and Spellbook of Fate. It can also keep Kycoo from messing with your graveyard in a pinch. However, Mistake's just so much better here. It can totally shut down the Spellbook player's early game. The best case scenario for them is to somehow get a Fate or Priestess play off. In the meantime they'll be mostly helpless, allowing you to set up an unbreakable position very early on. Since Mistake's a better card against Dragon Rulers and Spellbooks, it's entirely possible that it'll surpass Imperial Iron Wall in terms of popularity.
It's easy to make comparisons to Thunder King Rai-Oh, particularly because both cards contain similar text. There are some key differences in how each operates, and more than a few clear advantages in each card's favor. First, let's look at the fact that one of these cards is a monster and the other is a Continuous Trap. Monsters are at risk of being destroyed from a lot more than card effects--monsters with higher ATK are just as threatening to them. Rai-Oh mitigates this threat a bit with its 1900 ATK and Summon-negation effect, but it's still easy prey for Dragon Rulers. Nearly every deck plays monsters that are easily Summoned and big enough to take Rai-Oh out in battle, thus making it a bigger liability than it might be worth. Mistake's different: it's vulnerable to spell and trap removal but has nothing to fear from battle. Mystical Space Typhoon might be at three, but there's nothing stopping you from siding in two to three copies of the trap. That's something you just can't do with Rai-Oh right now.
The second important difference between the two is Mistake's chainability. Activating it in response to a discard with Dragon Ravine, a Dragon Ruler's effect, or a Spellbook is great. You can even chain it to Pot of Duality and prevent your opponent from adding cards. They'll still have to forgo Special Summoning for the turn, and any other search effects they may have wanted to use are now unavailable to them. You can quickly turn the tide in a duel with a well-timed Mistake activation. In the past, you could pull of similar tricks with Call Of The Haunted and Rai-oh, but Mistake performs the same function with one less card.
For early game disruption Mistake's incredible, but it won't do all the work for you. Dragon Rulers pack enough draw power to burn through their deck and grab Typhoons to answer your trap. Iron Wall typically prevents them from using Sacred Sword of Seven Stars, but Mistake doesn't do much about that. Combining the two together is greatly rewarding, and might be the best way to play them. As strategies continue to rely on searching the deck to make plays, Mistake will be a powerful Side Deck option against them.
Now I must ask if Borz works with mistake? Since Fresh suggested I add one. Cards such as maxx "c", reckless still work as I do comprehend.