http://newyork.cbslocal.com/2014/09/04/clinic-where-joan-rivers-went-into-cardiac-arrest-under-investigation/
"CONCLUSIONS : Most perioperative cardiac arrests were related to medication administration, airway management, and technical problems of central venous access. Improvements focused on these three areas may result in better outcomes".
http://www.ncbi.nlm.nih.gov/pubmed/12131111
With all due respect, she had clearly had quite a bit of work done over the years so there should have been pretty good medical history for the MD relative her body and administering anesthesia, making it more suspicious. Its not like she was in some rag tag office because they were the only ones who took her insurance or medicare.
On the conspiratorial side, I heard awhile back the new 'thing' was slipping a chemical into a target before a scheduled procedure that would prove fatal when general anesthesia is administered because of a severe drug interaction. Everyone then assumes some sort of medical error or unknown allergy as the cause and thinks nothing of it other than a tragic loss. Sounds very plausible, but I'm certainly not personally vouching for the info.