Fusel alcohol
Hot, solvent-like, and a burn at the back of the throat out of proportion to the actual alcohol.
What causes it
Fermentation run too warm, or too little yeast pitched into a high-sugar must.
Detection threshold
Noticeable in most people above roughly 300 mg/L of higher alcohols.
What you can do
Time, and only time. A year in bulk lets some of it esterify into something pleasanter, which is genuinely why big sweet wines improve.
How to prevent it
Control the temperature. This is the single most common fault in home mead and in high-gravity country wine.
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.
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.