Poor Food Choices

Bad Fruits & Veggies
NEVER feed them Spinach under any circumstances it inhibits their calcium absorption. Kale in recent studies has shown to have the same principles as Spinach so to be safe I would say no to that as well. Carrots (which should only be fed shaved or diced) can be fed but only on occasion the same with Bananas (which should only be fed diced into small pieces to avoid getting stuck in the throat). They can also alter a dragons calcium levels which can cause serious health problems.
Real Greens, Not Water
Just because it is a leafy green does not mean it is a good staple everyday green. The main types of lettuce that are often fed to dragons daily are Iceberg and Romaine. These have little to no nutritional value and are compared to eating water. Don't waste your money, walk a few steps over to another section at the grocery produce department and get Collard, Mustard, Dandelion or Turnip Greens instead.
Not a Super Food for Dragons
NEVER: feed a bearded dragon Avocado. Avocado are toxic to dragons and can cause death.
Stay Away from the Light
NEVER: feed a bearded dragon a firefly aka "lightening bug" and never take them outside when fireflies are out because they will eat them accidentally.  There is a chemical that is fatal to dragons called adenosine triphosphate in the insect, it is part of what makes the firefly glow. Please do not for any reason feed your dragon one of these insects, it will kill them.
Photos from google images
The photos below are from a little dragon that got very lucky and did live after accidentally eating a firefly, but only because its owners took it to an emergency vet right away.
I was given permission to post these photos by: Animal Emergency Center - Novi.
Unnecessary Feeders
Live feeders such as rodents and small lizards should NEVER be fed to dragons. Not only is there no valid reason for feeding a live animal to a dragon it can have very harmful effects. Live animals have bones, skin and muscle and do not break down with a few chomps like an insect. Bearded dragons cannot chew as they don't have teeth, only a serrated jaw line, and something this large could cause them to choke. And even if they manage to slowly swallow the animal it is very hard on their digestion and can cause punctures in the stomach and intestine. Aside from health risks feeding live animals there are no proven health benefits from feeding these to bearded dragons. And sadly it has been shown to make them much more aggressive as they learn other animals can and should be eaten.