The primary information we have about people deteriorating has been due to the attempted use of EMDR by untrained clinicians. Only recently, another instance was revealed--it was clear from the client's description that only my initial 1989 article had been used. In one case, a psychiatrist told his client that he'd gotten his training from watching the EMDR 20/20 segment! This is particularly problematic because all the media attention and misinformation in some of the published literature have led people to believe that all you do is move people's eyes around. That is very far from the case.
If the appropriate EMDR protocols are followed there is a thorough history-taking and preparation stage. EMDR should not be used until appropriate stability is attained. This preparation is vital. If there is any deterioration it would be unusual and quickly spotted. In that case, an appropriate assessment of cause should be done and then an intervention made until stabilization is achieved.
The research reports consistently show excellent results with EMDR if it is appropriately applied. Just remember, that during the initial phases of treatment the client may get in touch with very disturbing emotions and memories. This should be appropriately handled by an experienced and well-trained clinician regarding when to proceed, and when to augment or cease the present active treatment.