Maybe Hitchcock's Characters should Sweat the Small Stuff

At the beginning of North by Northwest, after Thornhill (Cary Grant) has rattled off instructions to his secretary, Maggie, in the taxi, he stops at the Plaza Hotel and steps out of the car. Just as it's pulling away, he calls out: "Oh, one more thing, your dad called...." or something to that effect. But the taxi pulls away before Maggie can get the message.

The otherwise sharp-witted, eye-on-every-detail Thornhill made a tiny mistake, which conceivably, set him up to be mistaken for Kaplan moments later.

If Thornhill had remembered those instructions, or even been able to stall the cab even for a moment while he delivered it, the fatal coincidence that occurred when he summoned the waiter (with a wave of his hand as if hailing a taxi) would possibly not have occurred. His fate lay in the misbegotten timing of a simple gesture. Then again, it was what those two gestures -- at the taxi, as he blithely commanded his secretary to do his lying for him; his casually authoritarian summoning of the waiter as if hailing a taxi -- represented, self-will run amok, that got him into hot water.

Micro-errors like this in Alfred Hitchcock's films occur at other times. At the beginning of Dial M for Murder, over breakfast, Wendice (Ray Milland) accidentally knocks over a salt-shaker. I've always seen this as foreshadowing in a small way Wendice's later small error in judgment that leads to the discovery of his guilt in attempting to have his wife killed. In Thornhill's case, however, his error puts him on a journey of self-discovery.