Off-flavors
Updated
The complaint that arrives as "it tastes wrong", and that is unanswerable without a vocabulary.
A vocabulary is worth having
"Off" is not a diagnosis. Naming the character narrows the cause immediately, and most off-flavors have a small number of possible sources.
The named ones
Vinegary: acetic acid, from acetic bacteria. Normal in small amounts in kombucha and a fault in anything else.
Solvent or nail-varnish: ethyl acetate, usually from a stressed or over-long fermentation.
Yeasty or beery: fermentation where there should be none.
Buttery: diacetyl, from bacterial activity.
Cardboard or papery: oxidation, the classic staling character.
Cooked or jammy: excessive heat, and unavoidable to a degree in a hot-filled product.
Metallic: metal contact, or oxidation catalyzed by metal ions in the water.
Medicinal or plastic: chlorophenols, from chlorinated water meeting phenolic compounds. A real reason to filter the water.
Where they come from
Three broad sources, and identifying which halves the search.
Raw materials: an oxidized oil, an old tea, a fruit that was past it.
Process: too much heat, too long an extraction, a fermentation that ran too warm or too long.
Storage: oxidation, light, time, or the package.
The timing tells you which: present at filling means raw material or process; developing later means storage or the package.
Diagnosing
Taste retained samples from the batch: one refrigerated, one warm, one from filling day.
Compare against a reference bottle from an approved batch at the same temperature.
Check the batch record for anything unusual: a longer hold, a different lot, a warmer room.
And keep a simple log of complaints with the batch code, because a single off-flavor is an anecdote and three from one batch is a cause.