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.
What causes it
Alcohol above the strain's tolerance, a temperature crash, nitrogen exhaustion, or a must so high in sugar that it inhibits the yeast osmotically.
Detection threshold
Two identical gravity readings three days apart, above the target.
What you can do
Warm it to 20C, stir gently, add nutrient, and if that fails build a starter of a restart strain and acclimatise it to the stuck wine in stages rather than pitching straight in.
How to prevent it
Match the strain's tolerance to the starting Brix before you pitch, feed in stages, and keep the temperature steady.
Others in fermentation
Hydrogen sulphide
Rotten egg. If left, it develops into mercaptans that smell of onion, garlic or burnt rubber.
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.