I don't want to be extremely categorical in answering that question, because there could be a lot of factors at play.
In a general way, a miscarriage of justice is often seen when any new information or evidence that comes to light calls into question the reliability of the verdict or the process that led to it. Sometimes that second branch—the process that led to it—could be things like coercion, the extraction of a false confession or threats. You gave the example of fabrication of evidence. That's an example.
There are also other circumstances. In the early 1990s, for example, a broad review was conducted of self-defence after the Supreme Court came out with its decision on Lavallee, with the battered woman's syndrome and all of that. A lot of cases needed to be examined just to see whether a valid defence of self-defence could have been advanced and was overlooked. That's despite it having resulted in, for example, the death of a spouse.
There could be a variety of different circumstances. We can't be very categorical, but that is also the benefit. We try to describe it as not being defined, but the courts have not had any problems, as I've observed in my reading, with wrapping their minds around the concept of miscarriage of justice.