Rolling before the wither has finished
Withering does two things that are usually described as one: it removes water so the leaf can be twisted without shattering, and it gives the leaf hours in which to change while it is still intact and alive. Roll too early and the leaf is still turgid, so it smears against the roller rather than twisting. Smeared leaf is ruptured unevenly — some of it comprehensively, some of it barely — and it then oxidises unevenly, so a single lot arrives at the drier containing both over-oxidised and under-oxidised material. The cup is simultaneously harsh and thin, which is a combination that reads as a bad garden rather than as a bad hour, and the garden gets the blame. In CTC manufacture the same error has a more visible symptom: the leaf will not granulate, and the output is a sticky mass that dries into flakes.
Applying heat too early, or too late, on the oolong branch
The oolong maker's only real decision is when to fix, and it is taken by smell in the middle of the night. Fix too early and the aromatic development that the standing rests were building has not happened: the tea is green and sharp, with a thin, grassy character and no depth, and no amount of roasting afterwards adds what was never made. Fix too late and the oxidation has run past the target, the liquor is heavy and amber where it was supposed to be pale gold, and a tea sold as a floral style tastes of stewed fruit. Neither is recoverable, and both are why oolong production cannot be scheduled: the rests lengthen as the night goes on, and a batch that has fallen behind stays behind. A related and worse error is shaking too hard too early, which ruptures the leaf centre as well as its margin. The leaf then oxidises through rather than at the rim, and what comes out is closer to a poor black tea than to any oolong.
Drying in one pass instead of two
This one is purely physical and it is the commonest corner cut in the industry. Surface moisture leaves a leaf quickly; moisture inside the leaf has to diffuse outward before it can evaporate, and that is much slower. Push the heat to finish the job in a single pass and the outside case-hardens while the inside stays wet. The tea reads as dry on the day, weighs correctly, and passes an inspection. Some months later it is stale, slightly sour or musty, and the buyer blames their own storage. The correct sequence is a hot fast first pass, then a rest of anything from twenty minutes to several hours during which internal moisture reaches the surface, then a cooler longer second pass. The rest is the step, and it is the step that gets dropped when a factory is behind.
Packing or rolling leaf while it is still hot
Every hot operation leaves the leaf carrying stored heat and surface moisture, and a leaf that is not cooled keeps cooking itself after the operator has stopped. Roll leaf hot from the pan and it stews: the fresh top note goes, the colour yellows, and a faint sourness appears that is easy to mistake for a fixation fault. Pack a finished tea hot from the drier and it holds moisture it should have lost, which shortens its life in storage by months. Hojicha is the extreme case — the leaf leaves the roaster with enough heat to keep roasting, and a batch that looked correct in the drum can be ashy by the time it is cool — which is why immediate forced cooling is a step in that route rather than a courtesy. The general rule is that an operation which is not stopped cleanly runs on into the next one, and the cooling is what stops it.
Doing the right step at the wrong point in the tail
Two examples, both from oolong and both invisible in a written recipe. Picking the stalk out of a strip oolong after the roast rather than before it means the stalk was roasted, and roasted stalk contributes a hollow, woody note that nothing removes afterwards. And judging a roast before its rest is the standard error in both directions: a tea straight off the charcoal tastes of charcoal, so producers who taste too early conclude they under-roasted and roast again, while buyers who open a tea too soon conclude it was burnt. The same tea, three months later, is noticeably different without anyone having done anything to it. Neither of these is a mistake in the operations performed; both are mistakes in when they were performed relative to each other.
The mis-orderings that show up years later
The pu-erh family is where timing faults have the longest fuse. A fixation run too hot destroys the residual enzymatic capacity that the sun-drying was there to preserve, and the resulting cake is drinkable, pleasant and permanently young; the fault is invisible for about five years and then it is simply an absence of anything happening. Drying maocha in an oven because the weather turned does the same thing more thoroughly. Pressing a cake and drying it in a heated room to keep to a schedule adds a second problem on top: the moisture that the steaming introduced leaves the surface faster than the core, and a cake stored with moisture still inside it grows mould from the inside out. By the time it is visible on the wrapper the tea is gone, and it will be blamed on the owner's cupboard rather than on a drying room they never saw.