In 1983 Paul Desmond Taafe imported certain packages into England. He thought they contained currency, which he erroneously believed was illegal to import. The packages actually contained cannabis, which was illegal to import. Was he “knowingly concerned in [the] fraudulent evasion” of any prohibition on importing goods?
He was convicted but appealed. “If we describe his action in terms of his own beliefs (about the facts and about the law), it obviously constituted an attempt to commit (indeed, it constituted the actual commission of) that crime,” writes R.A. Duff in Criminal Attempts. But Taafe wasn’t “knowingly concerned” in evading the ban on cannabis — he didn’t know he was importing cannabis. And however guilty he may have felt for smuggling currency, that wasn’t a crime.
He was acquitted.
(Taaffe  1 WLR 627 (CA);  1 AC 539 (HL).)