Gutshot
An inside straight draw: one specific rank fills it. Four outs, roughly 8% per card.
You're on the button with 9-8 offsuit. The flop comes Q-10-2 rainbow. Only a jack fills your straight: the board hands you no open-ender, just one rank slotting into the middle. Four jacks are left, so you're an underdog to hit on the turn, about one card in twelve.
That narrowness is the point. A gutshot is the easiest straight draw to hide. On Q-10-2 opponents read open-enders, not a single-card fill, so when the jack lands you get paid off by top pair or a set that never saw it coming. It also works as a semi-bluff: four outs plus a well-timed raise can fold out better hands even on the cards you miss, especially heads-up against someone who checked to you.
Price it before you chase it. Four outs is roughly 8% for the next card, outs times two. A half-pot bet lays 25%: you're calling half a pot into a final pot of two, and 8 does not clear 25. Even a quarter-pot probe lays about 17%. So a bare gutshot never calls on the current price; it continues when something else is paying, two live overcards, a backdoor flush riding along, position that closes the action, or implied odds you can actually name: a deep opponent with a hand that will pay the jack off when it comes. If none of that is true, folding isn't tight, it's arithmetic.
Two upgrades are worth knowing. J-9 on Q-10-7 fills with any king or any eight, a double gutshot, eight outs, open-ender strength wearing the same disguise. And check which end of the window is yours: holding 6-5 on 9-8-2, the seven you're chasing makes your straight and makes J-10 a bigger one. Drawing at the low end of the board is how a gutshot turns from a cheap surprise into an expensive second-best hand.
The beginner mistake is playing every gutshot like a real draw. Four outs is half an open-ender, and calling street after street to chase it bleeds chips too quietly to notice at the table: it only shows up later, in the session's arithmetic. Ask the two questions every time (what's the price now, and who pays me if it comes) and the gutshot stays what it should be: a cheap lottery ticket you mostly decline.