Excess diacetyl
Butter or butterscotch. Attractive in a barrel-fermented Chardonnay, wrong in a Riesling, and unpleasant above a certain point in anything.
What causes it
Malolactic bacteria producing it faster than the yeast reabsorbs it, usually because the yeast was racked away too early.
Detection threshold
About 0.2 mg/L in whites, higher in reds where fruit masks it.
What you can do
Leave it on the yeast lees and stir. Live yeast reduces diacetyl to compounds you cannot taste, which is the same trick lager brewers use.
How to prevent it
Do not rack off the gross lees until malolactic is finished, and sulphite promptly once it is.
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.
Geranium taint
Crushed geranium leaf, and once you know it you cannot miss it.