Geranium taint
Crushed geranium leaf, and once you know it you cannot miss it.
What causes it
Lactic bacteria metabolising sorbic acid. It happens when a wine is sorbated but not adequately sulphited.
Detection threshold
About 100 nanograms per litre. Extremely low.
There is no fix
None. The wine is finished.
How to prevent it
Never add sorbate without sufficient free SO2, and never add sorbate to a wine that has not completed or been prevented from malolactic.
Others in fermentation
Hydrogen sulphide
Rotten egg. If left, it develops into mercaptans that smell of onion, garlic or burnt rubber.
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.