Flavour Defects

Can be classified according to the flavouring system (lacks flavour or too high flavour, unnatural flavour), the sweetening system (lacks sweetness or too sweet), processing related flavour defects(cooked), dairy ingredient flavour defects (acid, salty, old ingredient, oxidized/metallic, rancid, or whey flavours), and others (storage/absorbed, stabilizer/emulsifier, foreign). Some details are given below. 

Flavoring System

Unnatural flavor - Caused by using flavours that are not typical of the designated flavour i.e. wintergreen flavour on vanilla ice cream. esp. vanillin

Egg: Caused by using too much egg in an ice cream that is not specified as a custard ice cream - resembles French vanilla ice cream . 

Processing

Cooked: Caused by using milk products heated to too high a temperature or by using excessively high temperatures in mix pasteurization. It can dissipate with time, the same as cooked defect in fluid milk. Sulfhydryl flavor: Caramel-like, scalded milk, oatmeal-like. 

Dairy Ingredients

High Acid: Use of dairy products with high acidity (usually due to bacterial spoilage) or holding mix too long and at too high a temperature before freezing. Acid/sour flavours are more rare these days due to the growth of proteolytic psychrotrophs during storage at elevated temperatures, rather than lactic acid bacteria.

Salty: Ice cream too high in milk solids-not-fat. Too much salt may have been added to the mix. High whey powder, or maybe salted butter used instead of sweet butter.

Old Ingredient: Caused by the use of inferior dairy products in the preparation of the mix. Powders made from poor milk or stored too long at elevated temperature or butter made from poor cream will contribute to old ingredient flavour. Unpleasant aftertaste.

Oxidized: Caused by oxidation of the fat or lipid material such as phospholipid, similar to fluid milk oxidation. Induced by the presence of copper or iron in the mix or from the milk itself. Mono-and-di-glyceride or Polysorbate 80 can also oxidize. Various stages - cardboardy, metallic (also described as painty, fishy).

Rancid: Caused by rancidity (high level of free butyric acid from lipolysis) of milk fat. May be due to use of rancid dairy products (pumping or excessive foaming of raw milk or cream) or to insufficient heat before homogenization of mix. See description of Lipolysis, especially the release of free butyric acid.

Others

Storage: Usually develops from "Lacks Freshness" and is most pronounced on ice cream which have been held in a stale storage atmosphere. Ice cream can also pick up absorbed volatile flavours from the storage environment (e.g., paint, ammonia, or in dipping cabinets - volatiles from nearby flavours.