Hitchcock's first big mistake?

In interviews, Hitchcock used to say that the death of the young boy in Sabotage as his first big mistake and the false flashback in Stage Fright was his second.

I disagree. I think his first big mistake was dubbing Anny Ondra in Blackmail.

Ondra was a fine actress (the best that Hitchcock worked with in his British period) and it is frustrating that we only have half her performance in this picture. There is a clip of her talking with Hitchcock which shows her English was OK and easily understood.

Of course, her accent would have sounded odd coming from the daughter of an East End tobacconist, but Hitchcock could easily have added a couple of lines explaining that she had been brought up by relatives abroad. Her accent would then have been no more out of place than the cut-glass, stage school tones of the actress brought in to dub her.

I suspect that Hitchcock simply relished the technical challenge of inventing ADR in his first sound movie but the cost of this decision is probably the awkwardness in the staging and pacing of some of the scenes as he tried to disguise the dubbing (which was done live on set).