What ingredients should be avoided in stock?
There isn't anything that is necessarily "bad" or should always be avoided in stock, but some ingredients have qualities you won't always want.


Dark greens (spinach, kale, etc) can make a stock bitter and of course greenish in color. Cabbage also can impart a overwhelming bitterness.
Potatoes can cloud a stock from their starchiness, so they are not good when you want clear stock for something like a soup or consomme.
Tomatoes may overpower flavors in a light stock, but are a critical component in most dark stocks (browning tomato paste improves the color)
Onion skins add a deeper flavor, but yellow or red skins can change the color of a light colored stock dramatically.
Skin and extra fat from the meat used is sometimes avoided to reduce the amount of skimming required later on (I personally don't skim, the extra fat doesn't bother me)
The bones of very oily fish (mackerel, salmon, and trout for example) are usually avoided because they can make a stock too strong in specific flavors to work in any other dish. Oily fish stocks also tend to have an unpleasant odor.