Mistakes, yes; but I think most mistakes come from the MISINTERPRETATION of the copious raw data generated in an ECG. The machine itself is sort of primitive but the subtle pattern variations really take an expert to call well.
I just LOVE it when my GP makes believe he knows what he's looking at when he runs his little cheapy version that generates something that looks like a 2 foot sales slip.
He sometimes says: "Hmmm!"
I've tried to get my head around interpreting an ECG and I am struck by the extraordinary difficulty...way too subtle for me; it's really a junction of science and art and it seems like it takes YEARS of experience to do it well!