OK, I can think of a ton of humorous responses to this...  

Anyway, is there a danger in getting a massage from someone who is not properly trained?  EG, can they mess up your back, neck or something else?

but seriously folks:
There are actually blood pressure issues for those who are fragile in that area but....you'd know about that already.

When in doubt ask your Dr. and if something doesn't feel good let your masseuse know immediately.  People generally think "no pain, no gain."  I believe the reverse is true with massage.  It doesn't have to hurt!

Contraindications for Massage
Fever/high temperature though illness
Intoxication
Suffering from food poisoning
Very high or low blood pressure
Recent surgery/injuries to the local area (swelling/bruising to local area)
Painful cysts
Psoriasis or weeping eczema
Chronic arthritis of the upper spine
Osteoporosis
Epilepsy
Severe Asthma
Chronic fatigue
Spondylitis or spondylosis in the neck
Disorders of the nervous system
History of thrombosis or embolism
Aneurosa (localised dilation of the blood vessels)
Any potential fatal condition, such as Cancer

They should never put direct pressure on your spine.  SOmetimes untrained persons can imbalance your muscles by releaseing or spasming muscle fibers.  Additionally, untrained persons can just give ya bad energy by messing with your chackras.  I am very selective when choosing a massage person.. its a very intimate and trusting thing.

yes, it is possible.  
Standard caution zones - over the kidneys, backs of the knees, inside of the elbows, abdomen and zyphoid process, thorax.

Other things to be concerned about when working with the untrained - low blood pressure, veicrose veins, open wounds or recent injuries and surgeries, arthritis and osteoporosis.  If you have any of these PLEASE make sure to see someone who is trained properly.

Usually, bodywork is GREAT, even when someone is untrained.  Just make sure they do not try to force the body beyond it's comfort levels.  Communicate!
: )