Troubleshooting

Start from the symptom; we'll walk you to the likely cause, the process article that explains it, and the ingredient (if any) that's the usual suspect.

Symptom	Severity	Summary
Beer is sour, has a pellicle on the surface, gushes on opening, or shows visible mould	high	Bacterial or wild-yeast infection. Confirm with the symptoms above (not from a beer that tastes "off" in some other way), then assess whether the batch is recoverable.
Fermentation appears to have halted before reaching target final gravity	high	Gravity stops dropping above the expected FG. Confirm by reading three days apart; if unchanged for 72 hours, fermentation is genuinely stuck.
Beer tastes or smells like butter, butterscotch or movie-theatre popcorn	medium	Diacetyl is a fermentation byproduct produced by all yeast strains. Healthy fermentation reabsorbs it; truncated fermentation leaves it in the finished beer.
Beer tastes or smells like cooked corn, creamed corn or canned vegetables	medium	DMS (dimethyl sulphide) is produced by the thermal breakdown of S-methylmethionine during malting and brewing. Long, vigorous, lid-off boils drive it off; short or covered boils trap it.
Beer tastes or smells like green apple or fresh latex paint	low	Acetaldehyde is an intermediate in normal yeast metabolism. Healthy yeast reabsorbs it; bottling too early traps it in the beer.