In the March/April 2006 edition of Zymergy, there is a very good article on fermentation flaws and how to avoid them. It is of course catered to beer brewing, but it is still relevant to Mead making as well. It discusses fusel alcohols, esters, diacetyls and sulfur compounds, how they are made, how to detect them, and what to do when you have them. Of particualr interest is the yeast pitching rate, and how too much or too little can create problems with flavor and the creation of unwanted products.

If you can, pick this edition up, or borrow it. Well worth the read.