Almost any legal or factual error that you think was committed in your case at or before your trial can, in theory, be raised on appeal. However, most often, the most likely issues to be considered and granted relief on appeal are errors of law (as opposed to errors of fact). Errors of law are errors that a judge made in your case – i.e. whether certain rulings that the judge made were correct. For example, the decision not to grant a motion to suppress or a decision to permit certain testimony over the objection of defense counsel are potential errors of law.
Conversely, errors of fact, i.e. decisions by the jury about what happened in the case, can also be overturned, but such instances are rare. An example of an alleged error fact would be to say that there was no evidence to show that the defendant intended to distribute to others the small amount of drugs found on his person.