You spent hour after hour planning out your weightlifting program. You sought out the latest information, researched the latest and best programs, and when you got to the gym, you attacked every muscle fiber from the optimal angle.
But when it comes to cardio? Bah. Cardio is cardio. It's a necessary evil you toss in at the end of a workout, and the specific variety doesn't matter so much. What matters is what's on TV while you do it. Right?
Sadly, this mindset leads many people astray. Not only are they not doing the most effective cardio training they could be, but they also put themselves at risk for overuse injuries and muscle loss. On the other hand, if you stay mindful of both the quantity and quality of the cardio you do, and it can make all the difference in your energy level and appearance.
Let's go over some of the top cardio training mistakes many weightlifters make, so you can make sure you don't follow in their footsteps.
Mistake 1 /// Never Incorporating Variety
When it comes to cardio—perhaps even more so than weights—every
person tends to have their favorite and least favorite piece of
equipment. Maybe you're a runner at heart and head for the treadmill
with joy. Or perhaps you wear a "Running Sucks" T-shirt and favor
cycles, the elliptical machine, or the stairclimber instead. Whichever
implement you choose, you most likely return to it day-in and day-out. 
If your goal is to improve your performance in preparation of the first annual World Championships of Stair Climbing, this is a great plan. On the other hand, if your plan is to see fat loss or overall fitness improvement taking place, you'd likely benefit from mixing it up a little.
As your body gets used to the movement patterns it performs on a regular basis, it gets better at them. This means you won't expend the same degree of energy as before. Adding some variety keeps your body guessing, so you can continue to see results.
You wouldn't perform the same weight routine for months on end, would you? Of course not. So don't do the same with your cardio.
Mistake 2 /// Doing Interval Training Fasted
[Sound of alarm clock] Good morning world! No breakfast for me, thanks. I'm off to do hill sprints!Whoa, slow down there. You probably heard by now that fasted cardio training first thing in the morning is excellent for fat loss, and that interval training generally beats out steady-state cardio training. Put two and two together and it's sure to be the most unbeatable cardio program ever, right?
Unfortunately, it doesn't always work this way. Most bodies need to have glucose to burn as fuel in order to exercise at such high-intensity levels. For them, attempting fasted sprints is a recipe for disaster. Not only will their performance suffer, but they also set themselves up for lean muscle loss.
Some low-carb advocates will argue that athletes whose bodies are in a state of ketosis—and well-adapted to it—can train at high-intensity without any performance setbacks. That may be true, but the rest of us should keep fasted cardio to moderate intensity levels.
Mistake 3 /// Relying on Cardio Training to Create the Deficit
This may not be news to you, as a Bodybuilding.com reader, but many
in the gym-going population at-large need it to be repeated. So here
goes: Cardio is not enough!
Many people figure that since cardio is difficult and it makes them
sweat, it's all they need in order to create the caloric deficit
necessary for fat loss. By this rationale, they do more and more cardio,
figuring each minute is one minute they don't have to spend watching
what they eat. This is a great way to overtrain your body and simply become exhausted. Remember, the body can only handle so much intense exercise of any type. Then it needs time to recover. Otherwise, you'll suffer overuse injuries, burnout, your sleep and immune system could suffer, and you may lose muscle mass as well.
A tip: If you "have to" do an hour or more of cardio per day just to burn off the extra calories you consumed, the answer is to look at your diet. You can spend all day on the treadmill and never outrun your bad choices.
Mistake 4 /// Using Light Weights During Cardio Workouts
Ever watch someone perform their cardio workout while holding 2- or
5-pound dumbbells or tiny little wrist weights? Of course you have.
There they are, doing their biceps curls, lateral raises, and overhead
presses, all while running, jumping, or convulsing in some other way. 
You deserve better! These movements do little to strengthen the
muscles you're trying to work, and the measly calories you burn aren't
worth the time it takes to pick them up off the ground.Are they harmful? Probably not, but they take away from the intensity you could otherwise be applying to far more effective cardio or weight training.
One exception to this rule may be for those who like—if that's the right word—to wear weighted vests while running or performing other cardio. Since this evenly distributes weight over the body and doesn't require any additional movement, a weighted vest can increase your cardio fitness level and caloric expenditure.
However, you need to be pretty tough and already in shape in order to use it properly and safely. Don't rush it.