These are some of the common mistakes people make
Red Lights or Night Lights - Bearded dragons are diurnal, which means they are away during daylight hours and sleep in the dark at night. Dragons also relate heat to bright white light like the sun and will naturally be encouraged to bask in bright light. Dragons do not need any kind of light at night and only need to stay warm at night if the temperature drops below 70 degrees. During winter, a ceramic heat emitting bulb can be used at night to keep the temperatures from dropping too low.
Quality UVB Lighting - Just because a bulb is marketed towards reptiles and says it has UV doesn't mean it is safe to use or will supply enough quality UVB to keep a dragon healthy. ALL bulbs have UVA, even common household bulbs, any bulb that states that it has UVA or is for basking is only made for providing HEAT for basking reptiles. Since all reptiles depend on their environment to provide body heat, basking bulbs are needed to facilitate metabolism and proper digestion. Most dragons will seemingly live just fine with only a basking bulb, but without the correct UVB, they will end up with calcium deficiencies. Calcium deficiencies will cause stunted growth, poor skeletal formation, and can cause poor muscular and nervous system function. By the time symptoms become apparent, the health of the dragon has been seriously compromised.
Sand Substrate - Many people believe that because dragons are from the dessert, sand is common in their natural habitat. Dragons are from dessert regions where sand is packed down hard like rock, not from desserts like the Sahara where sand dunes are everywhere. While play sand may not contribute to impaction like dangerous calcium sands marketed for reptiles, all sand houses bacteria and has dust that can contribute to upper respiratory problems and yellow fungus disease. Sand should be changed completely at least monthly to even assume it is relatively clean.
Community Housing - Many people mistake dragons for community animals and think they need a friend to keep them from being lonely, this could not be further from the truth. It is common to see babies piled on top of each other as if they "like" each other. This is merely a display of dominance and a fight to get the best perch. In the wild, the most dominant dragon has the highest perch, so babies are always fighting for the upper hand. In an enclosed environment, there is no way for a submissive dragon to get away or escape a dominant dragon in an adverse situation, so it is never fair to assume any dragons will successfully be housed together in a tiny 18" x 36" tank or even a 24" x 48" area. Housing dragons together past the age of three months is risky and will almost always result in violence unless you are lucky enogh to get two docile females. Never assume two dragons of the opposite sex will get along nicely and NEVER put a smaller dragon in with a bigger dragon!
Not Eating Greens - Many people have dragons that will not eat their greens, which is known to cause shortened life span due to untimely organ failure. Dragons are not like dogs and cats, they DO NOT need to constantly be fed in order to keep them alive. The reason reptiles have evolved and been on this planet for thousands of years longer than mammals is due to their slow metabolism and ability to conserve. A healthy dragon can go for very long periods without food, so going a couple days or a week is not going to hurt them. If a dragon over the age of 3 months doesn't eat it's greens, resist the urge to feed it bugs for at least one or two days. For older dragons it may take a week or longer, but if it gets hungry enough, it will usually eat its greens. You can also try different kinds of greens or hand feeding to get their attention easier.
Hydration or Dehydration- Baths are not always enough, a dragon must drink a certain amount of water or get moisture and electrolytes from their greens every day. People assume that spraying and soaking will keep their dragon hydrated, but it doesn't mean the dragon is actually drinking enough. Water alone isn't always enough to properly hydrate by itself anyways, a living body cannot use plain water without a correct balance of electrolytes. This can be especially dangerous for smaller dragons or babies who may not be eating due to relocation stress and can be fatal if not treated. A dehydrated dragon will not eat, not eating causes further dehydration, add too much or not enough heat and then it won't be long before the kidneys start to fail. Forcing fluids with electrolytes in small amounts throughout the day becomes paramount in this situation and will save a dragons life.
Impaction - many people cannot risk the temptation of feeding mealworms because it pleases them to see their new dragons eat. Many babies go through a period of relocation stress when they first arrive in their new homes and their new owners become impatient when they don't eat right away. Most pet stores will recommend mealworms and most dragons will readily eat them. The problem with mealworms is their exoskeleton or chitlin is not easily digested and builds up in the intestines causing blockage. This is almost always fatal even when caught early and treated by a vet.