KGB Warning — This is a caution given by police, to a person giving a statement that may be used later in court. It creates a possible exception to the hearsay rule.
The Supreme Court of Canada case that raised the issue is R. v. B.(K.G.),  1 S.C.R. 740. Initials are used because the accused was a minor. It has come to be known as “KGB.”
Before the ruling, prior inconsistent statements made by a witness other than an accused could be used only to impeach the witness’s credibility. Now, if the statements are necessary and reliable, and they meet other criteria, they can be admitted for the truth of their contents. To be admissible, the statement must have been made under oath, solemn affirmation or solemn declaration, after an explicit warning to the witness that making a false statement is a serious criminal offence.
Related Cases & Information:
K.G.B. Statements, Unsavoury Witnesses, and Vetrovec Warnings — CanLII Connects summary by Jonathan Melo
R. v. Toten, 1993 CanLII 3427 (ON CA)
R. v. Finta, 1994 CanLII 129 (SCC),  1 SCR 701