Too Much Cardio
I wrote an article pointing out how too much cardio can hurt your fat burning efforts.
Not Organizing Exercises in a Workout Properly
Not organizing exercises properly can make the workout inefficient and less effective in achieving your goals.
Not Keeping Track Of Workouts
This is a mistake that a lot of people make. How are you supposed to know if you're improving if you don't keep track? You can actually be doing less work in successive workouts. I now keep track of every single one of my workouts as well as each of my clients. If you need help with this I would recommend downloading the Fat X Workout Guide.
Concentrating Too Much on Quantity of Calories vs Quality
This is actually one of the biggest mistakes most people make. Foods don't just contain calories but they provide other nutrients that benefit your body and some foods have the lack of these nutrients. Those nutrients can directly and indirectly help in burning off excess fat.