The value of physical evidence, and any associated forensic testing, hinges upon the proper collection, labelling, record-keeping and storage of the evidence by the police in advance of trial.
The police must preserve the integrity of a seized item and ensure that it is not altered or lost.
The police must also maintain a reliable ‘chain of custody’, including details as to who had access to the evidence and for what purposes.
The mishandling of physical evidence may jeopardize a prosecution in several ways, including:
1) The police may be unable to locate (or may prematurely dispose of) the physical evidence, such that it is not available to the Crown for presentation at trial;
2) The Crown may be unable to persuade the Judge that the exhibit at trial is the same item that the police seized during the investigation; and
3) The Crown may be unable to persuade the Judge that the item that the police sent for forensic analysis is the same item that the police seized during the investigation. (See R. v. Tran, 2013 ABPC 312 – A case involving a fatal missing link in the chain of custody in a drug investigation before the seized substances were sent to the drug analyst.)
The prosecution has a duty to disclose all relevant information to the Defence (See R. v. Stinchcombe,  3 S.C.R. 326). The Crown must disclose any known deficiencies with respect to the handling of physical evidence in a particular case.
In the absence of any information that suggests a problem with respect to the handling of physical evidence, it is common for the Defence to concede the continuity of that evidence and to focus on other issues.
When the disclosure materials reveal a potential issue with respect to the chain of custody or counsel is aware of systemic problems with the handling of physical evidence by a particular police department, counsel will politely decline the Crown's request to admit continuity during pre-trial discussions.