Best data I can find says it's been on the order of >20 <100 per year over the last ~30 years. Which seems relatively reasonable given the size the denominator. Wrongly deporting legal residents and citizens is obviously awful but there is no such thing as a perfect process.

Hard to tell since the legacy media has historically leaned left (and has tended tend to look the other way on stories that make democrat administrations look bad), but I would not be surprised if, out of 7 million, some % of corner cases slipped through due to human error.