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Injectors were moved between cylinders and the calibration data was not moved with them.

Swapping an injector to a different cylinder is a legitimate and useful test — if the symptom follows the injector, the injector is the problem; if it stays with the cylinder, it is not. The mistake is not the test but what is skipped afterwards: each injector's correction code is bound to the cylinder it was entered against, not to the injector, so moving the hardware without re-entering the codes leaves every moved cylinder running on someone else's data.

The outcome is worse than the fault being investigated. Before the test, one cylinder was suspect. Afterwards, two are being fuelled wrongly and the original symptom may have moved, stayed, or changed character — so the test's result is no longer trustworthy either. A diagnostic swap that is not accompanied by a coding swap destroys the evidence it was performed to gather.

It happens most often when injectors are removed for another reason entirely — a head gasket, a rocker cover, seal replacement — and are refitted in whatever order they came to hand. There is nothing physical to prevent it and nothing to show for it afterwards, since all the injectors are identical parts.

The fix is not mechanical: read back the stored codes, compare them against the labels on the injectors actually fitted, and correct the mapping. Nothing needs to come apart again.