Prednisone does have some mental effects but I don't think hallucinations is a recognized side-effect. I could maybe see in rare cases a very severe prednisone-induced mania resulting in hallucinations though. Do remember that people can have atypical adverse reactions to any medication though.
When I was getting my CMA my teacher explained that her husband has an atypical reaction to Robitussin for example. His reaction was severe hallucinations. He basically had an acid trip on an over the counter cough suppressant.
My wife has an adverse reaction to benadryl where it causes her to have an anxiety attack without fail every time.
These are not side-effects in the normal sense or allergic reactions as they are not an immune reaction. It's simply due to tiny genetic variations. One gene in the wrong place can mean that a certain chemical is dealt with by your body in a completely different way than everyone else.
Now to answer Is1krab's question.. diabetes, severe weight gain, swelling (sometimes to dangerous levels), psychological issues of many kinds, unusual fatigue and muscle weakness (when I was on 40mg I couldn't get out of a chair without using my arms), infections that can at times become very severe (a simple flu earned me both bronchitis and a sinus infection that I'm just now getting over), osteoporosis (start worrying about breaking your hip 20 or 30 years early), and cataracts just to name a few.
