May the record reflect...

On appeal, the record is your world. If it's not in there, it didn't happen in the appellate universe--even if it did happen in real life. Be sure you know when you start whose responsibility it is to ensure an adequate record on appeal (in federal court, both parties and the trial court have record-related duties; in some state courts, the appellant alone bears the burden). Knowing this can make the difference between winning and losing, because a presumption of regularity is going to attach--basically, an assumption that all went according to the law absent a record to the contrary--and fall on the party bearing the burden of ensuring record adequacy.

Often times, things happen in court that for some reason or other do not make it into the record--a sidebar conference is too muddled for the reporter or recording equipment to pick up; the exhibits are returned to the parties for some reason; a chambers conference doesn't get recorded or summarized on the record; etc. An unsophisticated practitioner will refer to those things in his brief without ensuring they are part of the record. Be aware that there are rules to help complete the record in situations like this (like Federal Rule of Appellate Procedure 10(c)-(e) or comparable state rules, see, e.g., Utah Rule of Appellate Procedure 11(h)). Some states also allow supplementing the record with facts that never happened in court, but bear on a discrete appellate issue like an ineffective assistance of counsel claim. But however your jurisdiction allows for it, be sure to follow the procedures rather than (as I've seen from time to time) attaching things as addenda to a brief and expecting that the court will consider them.