What is the biggest mistake a lawyer handling an appeal can make in his or her appellate brief and/or oral argument?

Assuming that the brief has properly raised the issues and properly cited pertinent cases, the biggest mistake a lawyer can make is attacking the opposing lawyer. Sometimes it seems to be a sport, both in appeals and trials, for lawyers to focus anger on the opposing counsel, to ridicule or criticize or suggest he or she was untruthful, etc.