Where the Crown relies on expert evidence concerning a physical exhibit that has been destroyed before trial, the trial judge must give a clear and firm warning to the jury about the substantial disadvantage to the accused in being unable to have the exhibit independently examined and tested, and that it would be dangerous to convict on that evidence without scrutinising it with great care. The judgment provides a detailed model direction. Jones v Dunkel directions in criminal trials must be approached with marked caution and should not be given where they risk reversing the onus of proof, where the witnesses could not realistically be called, or where insufficient attention has been paid to what useful evidence the absent witnesses could actually provide.
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