A bike can shift perfectly and have no working limit screw at all. The cable is holding the derailleur, not the screw.

Set the limits with the cable out of the equation, not by shifting

The most important thing on this page and the least known. Park Tool explains why testing by shifting proves nothing: many people think the high limit screw is set appropriately, when it is actually the cable pull stopping the derailleur rather than the screw, and when you shift out to the smallest cog it appears as though everything is lined up as it should be. Their conclusion is that you will not know until you take the cable out of the equation, which is done by backing off tension with the barrel adjuster. Two different things can stop the derailleur at the same point and only one of them is the safety stop. A bike that shifts perfectly can have a limit screw set nowhere near where it needs to be, and it will behave impeccably right up until the cable lets go.

Limit screws are a fence. The barrel adjuster is the tuning

Keeping these separate prevents the single most common mis-adjustment, which is reaching for a limit screw because the shifting is hesitant. The limit screws exist to stop the chain going into the spokes or into the frame, and guidance is explicit that they are not part of indexing, that they must be set first, and that once they are right you leave them alone because indexing happens entirely with cable tension. So hesitant shifting is a cable tension question and a dropped chain is a limit question. Using a limit screw to chase a shifting complaint may appear to help and it does so by moving your safety boundary, which trades an annoyance for a hazard.

A bent hanger is unwinnable, and there is a pattern that identifies it

Worth checking before anything else, and one workshop puts the reason memorably: a bent hanger makes a perfect adjustment impossible, and you cannot perfect the imperfect. Another warns you will be forever chasing your tail with the indexing if the hanger is bent. The useful part is a diagnostic pattern that distinguishes it from a tuning problem. An adjustment issue shows as hesitation consistently in one direction across the entire cassette, or a chain dropping off the smallest or largest cog, or trouble isolated to one or two specific gears. A bent hanger shows differently: shifting works well in some parts of the cassette and poorly in others, no barrel adjuster position produces clean shifting across all gears at once, and the problem appeared after a crash or an impact. That second pattern means stop adjusting.

Cable stretch is not stretch, and a new bike going vague is expected

Useful both as reassurance and as a maintenance expectation. The phenomenon gets called cable stretch, and as one manufacturer explains, the cable is not really stretching, it is settling into the housing, and as that happens it is common to need to tighten the cable to preserve smooth shifting. So a brand new bicycle that shifted crisply on handover and goes slightly vague after a few rides has not been badly built, it has bedded in. The fix is a small amount of tension at the barrel adjuster, which is why leaving adjustment range available matters: reset the barrel fully clockwise then back it out two full turns so there is room to move in both directions later.

B-tension has two opposite symptoms, and it interacts with everything else

The adjustment people skip because it is less obviously connected to shifting. B-tension sets the gap between the upper guide pulley and the cassette teeth, and it fails in both directions with different signatures: set so the pulley is too close and the chain makes a rumbling sensation as you ride, set too far away and the shifting goes sloppy. Modern twelve and thirteen speed groupsets are described as particularly reliant on getting it right. It also interacts, so if the B-gap has been changed significantly it is worth re-checking the indexing afterward. And there is a condition specific to full suspension that is easy to miss: set the B-gap with the shock under its required sag, since the geometry the derailleur sees changes when the rider is on the bike.

Clean and inspect before adjusting, because dirt imitates bad indexing

A sequencing point that saves an hour of chasing a phantom. Guidance advises starting with a clean drivetrain because dirt buildup can mimic poor indexing symptoms, and then checking the specific wear items before touching an adjuster: frayed or rusty cable creates resistance, cracked or corroded housing ruins indexing accuracy, and a worn chain will not shift smoothly regardless of adjustment. Every one of those produces symptoms that read as a tuning problem, and none of them responds to tuning. Fifteen minutes with a rag and a chain checker in front of the job is cheaper than an afternoon at the barrel adjuster afterward.

Know the order, and know that it is a sequence rather than a checklist

A workshop sequence that works: confirm the hanger is straight, set the H limit with cable tension relieved so the upper pulley sits under the smallest cog, add cable tension at the barrel adjuster a quarter turn at a time until each shift lands clean, set the L limit so the chain reaches the largest cog without diving into the spokes, then set B-tension. Sources differ on exactly where B-tension belongs, with some putting it early and some last, and the honest reading is that it interacts with indexing rather than sitting cleanly at one position, which is why re-checking after changing it is recommended. What is not negotiable is that the hanger comes first and the limits are set before the tension.

Know when to stop, and shift through everything twice before you do

Two closing rules that are worth as much as any adjustment. The stopping rule: if you have set the limits, fitted a fresh cable, confirmed the hanger is straight and it still will not index cleanly, stop adjusting, because a persistently poor index after all that usually points to a worn drivetrain, a tired derailleur clutch or spring, or a hanger subtly bent beyond eyeball tolerance. A shop has an alignment gauge that removes the guesswork. And the acceptance test: shift across every cog twice before the next ride, because if it works only once through the cassette it still needs attention. One clean pass is easy to achieve by accident, and two is not.