The short answer is when there is spoliation.  Spoliation is the intentional or negligent destruction of evidence.  It may take place prior to, or during litigation, and it always deprives one side of the use of otherwise admissible evidence. 

In the helicopter case, it seems to have been intentional. IN RE HELICOPTER CRASH NEAR WENDLE CREEK, BRITISH COLUMBIA, ON AUGUST 8, 2002; Docket No. 3:04md1649 (SRU)
UNITED STATES DISTRICT COURT FOR THE DISTRICT OF CONNECTICUT; 2009 U.S. Dist. LEXIS 41477; May 18, 2009,

Spoliation comes into the legal malpractice picture when, for example, defendant attorney fails to take discovery of important evidence in the underlying case, and the ability later to obtain that evidence no longer exists.  In a matrimonial legal malpractice case, it may no longer be possible to discovery the other spouse’s financial records, and so plaintiff cannot show exactly how much she lost by target attorney’s negligent failure to take discovery.

In the helicopter case: "…defendants’ alleged failure to maintain or produce the allegedly "missing" records materially impaired her prosecution of her medical negligence and informed consent claims.

Plaintiff’s "negligent spoliation" claim is akin to a legal malpractice claim [*6] in that "damages arise from the loss" — or diminution of value — of an underlying claim. . . . [P]plaintiff’s primary medical negligence and informed consent claims ultimately failed for lack of proof of scientific/medical causation. Plaintiff argues that, if the allegedly absent records had been created or maintained and produced, Williamson might have been provided with the "missing link" that would have enabled him to identify and persuasively explain the causal relationship between gadolinium extravasation and Raynaud’s syndrome. Specifically, plaintiff points to the fact that no records reflect the amount of gadolinium used during the procedure. . . .