By way of example, the Court asked, sua sponte, at oral argument whether a large law firm we represented had any partners that were citizens of a state far removed from the state of the defendant's citizenship. Although jurisdiction was not contested by the defendant, at either the trial or appellate levels, the Court required the preparation and filing of an affidavit establishing the citizenship of each of the firm's over 200 partners. This, however, is not uncommon. I find that failure to make an offer of proof is another common error. I think the most common error I see in Missouri and Kansas is failure to make a proper offer of proof to preserve the error for review.
Many attorneys fail to make any offer of any kind.
More common is that the attorney merely recites into the record what he or she expects the evidence to be. Neither preserves the issue for appeal. At the very least, sworn deposition testimony must be read into the record. Better practice would require putting the witness on the stand for testimony outside the hearing of the jury and offering exhibits. Only then can the appellate court judge whether the error was prejudicial and possibly reverse.