I’m not sure I’m advocating it, but I’ve been playing a fun-off [scryfall]Hijack[/scryfall] in my Gruul Midrange sideboard. Why? I thought it would be a good finisher against Gate-decks. Stealing [scryfall]Gate Colossus[/scryfall] is a strong move against those decks, but admittedly, often unnecessary rub-ins instead of actual winning move, since it’s often a good match-up anyhow.
But it does tend to lead to weird situations. Especially in mirrors (or semi mirrors), where the opponent tries to go bigger and grindier.
For example, stealing opponents [scryfall]Treasure Map[/scryfall] was fun. Sure, you have to be careful here. It has to be on two counters and your opponent needs to have tapped it or be out of mana. Then, you can steal it and flip it. You don’t get to keep the land, but you get to keep the tokens, meaning that the land is less usable, even if it still produces mana. At least it’s three less cards for your opponent.
Another time the opponent broke a boardstall with [scryfall]The Immortal Sun[/scryfall]. Suddenly everything he had was bigger than my creatures, but gladly, they were lower on life, so just stealing the thing forced them to make a bunch of chump blocks and bad trades, which left me able to win on the following turn.
I have also been able to pay for [scryfall]Growth-Chamber Guardian[/scryfall]’s adapt by stealing an opponent’s treasure, although you shouldn’t try this, as they can just sac it in response (I had cast the Guardian with [scryfall]Unclaimed Territory[/scryfall]). It just happened that they didn’t think about it. This was quite risky and in the end it didn’t pan out, but I was able to slow my opponent down for a few turns.
How about this one: Opponent had [scryfall]Vaevictis Asmadi, the Dire[/scryfall]. I [scryfall]Hijack[/scryfall]ed that, targeted itself with it’s ability and got a [scryfall]Skarrgan Hellkite[/scryfall] for my troubles while opponent’s [scryfall]Demanding Dragon[/scryfall] became a Mountain.
However, my favorite is this one: I was playing a semi-mirror and we both had [scryfall]Rekindling Phoenix[/scryfall]es in our decks. Mine had died after [scryfall]Collision // Colossus[/scryfall] and [scryfall]Goblin Chainwhirler[/scryfall], but not to worry, I was able to steal my opponents one, attack with it and then [scryfall]Lightning Strike[/scryfall] to get myself the token. Opponent tried to save it with [scryfall]Collision // Colossus[/scryfall], but gladly I had my own and was able to still kill it.