The Court in Business Development Bank v 1956680 Alberta Inc, 2021 ABQB 141 described the plea of non est factum.
The plea of non est factum was explained by the Supreme Court in Marvco Color Research Ltd. v Harris 1982 CanLII 63 (SCC),  2 S.C.R. 774 at page 778:
... where a document was executed as a result of a misrepresentation as to the nature and character and not merely its contents the defendant was entitled to raise a plea of non est factum on the basis that his mind at the time of the execution of the document did not follow his hand. In such a circumstance the document was void ab initio.
 The Supreme Court in Marvco observed at page 780 that the plea of non est factum sprang into prominence with the judgment of Foster v MacKinnon (1989), L.R. 4 C.P. 704 which provides an illustration of the classic situation in which the plea can be raised. In that case the Defendant signed a document thinking that it was a guarantee when in fact it was a bill of exchange. The Court concluded that the Defendant never intended to sign a bill of exchange but intended to sign a contract of an entirely different nature. As a result, the plea of non est factum was successful.
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