* Scoring mistakes: Human error in grading the essay portion or misreading the scantron. While largely automated, the process isn't perfect.
* Scanning errors: Problems with the machine that reads the scantron could misinterpret marks.
* Data entry errors: Mistakes could happen when transferring scores from the scanning machine to the database.
If you suspect an error, you should contact the College Board directly. They have procedures in place to investigate score discrepancies and, if a mistake is found, will correct it. Providing evidence of the suspected error will be helpful in their investigation.