But sometimes, the case would have come out the same way even if the error hadn't been made. For example, the decision was made to allow some videotaped evidence that is later found inadmissible by the appellate court. The videotaped evidence was by a shopkeeper who testified that he had seen the defendant in his store at the time a crime took place. If there were six other reliable witnesses who testified that the defendant was in the store at the time a crime took place, it won't really affect the outcome if the shopkeeper's evidence is omitted. The error is harmless.
Trials cost a great deal of money.
There is no purpose to remanding a case for retrial, if the error was harmless. This is always a judgment call. In the example, I gave you it seems self-evident. Harmless error rarely comes up in point rounds. But some attorneys have raised it.