5 points worth checking on coffee brewing mistakes

Diagnosis is a skill separate from tasting. It means noticing what is wrong, then working backward to the stage that caused it. Most home coffee problems fall into four categories, and roast faults are the one people misattribute most.

What to take away

Under-extraction and over-extraction taste different in specific, learnable ways.
Roast defects have their own signatures and cannot be fixed by brewing.
Espresso aroma is dominated by extraction temperature and particle size, not by the machine.
Test one hypothesis per morning.

The four categories

What you taste	Category	Where it happened
Sour, thin, quick finish	Under-extracted	Your brew
Drying, hollow, harsh finish	Over-extracted	Your brew
Flat, papery, no aroma	Stale	Storage or roast date
Ashy, burnt, or grassy and vegetal	Roast fault	The roastery

The fourth row is the one to learn, because no adjustment you make will help and you will waste a week trying. The other three all have cheap and reversible fixes, and bypass dilution is the fastest of them.

Roast faults have signatures

Roast defects are specific rather than a general sense of wrongness. Work roasting beans from one origin under six time-temperature profiles identified marker compounds for five common defects, and found elevated indole for a light defect, 4-ethyl-2-methoxyphenol for scorched, phenol for dark, maltol for baked and 2,5-dimethylfuran for underdeveloped coffee.

You are not going to run mass spectrometry in your kitchen. What matters is the principle: a scorched roast, a baked roast and an underdeveloped roast are different faults with different causes, so "it tastes burnt" and "it tastes like cereal" point at opposite ends of the roast.

Baked coffee, the most commonly misdiagnosed, tastes flat and slightly sweet in a papery way, with very little aroma. People usually blame their grinder for it. If the coffee changed and the grinder did not, look at the bag before you look at the burrs.

If you brew espresso

Two variables dominate, and neither is the machine's badge. A review of espresso aroma concluded that extraction temperature and particle size were the main variables affecting the aroma profile, with extraction time also significant, and that furans are the most abundant class while sulfur compounds carry much of the characteristic coffee flavor despite low concentrations.

So diagnose an espresso through grind and temperature before considering anything else.

The diagnostic routine

Smell the dry grounds. No aroma means stale, and you can stop there.
Brew and note the time. Long and slow means fine grind or a clogged bed.
Taste hot, then again as it cools. Faults get louder as coffee cools.
Write one sentence naming the category, not the cause.
Change one thing tomorrow and repeat.

Naming the category before the cause is the discipline that makes this work. "Over-extracted" is a category. "My grinder is bad" is a conclusion you have not earned yet, and once you have written it down you will start finding evidence for it.

Keep the notes short and keep them in one place. Five words a morning for two weeks is enough to see a pattern, and a pattern is what separates a fault from a bad day.

When to blame the coffee

If the same coffee produces the same fault across two grind settings and two brew methods, the fault came with the bag. Buy a different one and see whether the fault travels. If it does, the problem is your process, and the ranked list of common errors is the place to start. If it does not, you have a roast fault, and it is worth telling the roaster, since some of them will want to know.