Hydrogen sulphide
Rotten egg. If left, it develops into mercaptans that smell of onion, garlic or burnt rubber.
What causes it
Stressed yeast, usually short of nitrogen, producing sulphur compounds during fermentation.
Detection threshold
Very low. Most people detect H2S at a few micrograms per litre.
What you can do
Splash-rack early and it often blows off. Once it has become a mercaptan it is far harder, and copper additions are a last resort with their own risks.
How to prevent it
Adequate yeast nutrient, added in stages, and a fermentation temperature the strain can actually cope with.
Others in fermentation
Stuck fermentation
Nothing in particular. It is diagnosed with a hydrometer, not a nose, and a wine sitting sweet and quiet for a week is stuck rather than finished.
Fusel alcohol
Hot, solvent-like, and a burn at the back of the throat out of proportion to the actual alcohol.
Excess diacetyl
Butter or butterscotch. Attractive in a barrel-fermented Chardonnay, wrong in a Riesling, and unpleasant above a certain point in anything.
Geranium taint
Crushed geranium leaf, and once you know it you cannot miss it.