Court reporters are certified at either 95% or 98% accuracy, depending on their certifying organization; however, the measure of accuracy is not one that evaluates their ability to transcribe nonstandard dialects.
In an experiment, the authors found that Philadelphia court reporters consistently fail to meet this level of transcription accuracy when confronted with mundane examples of spoken African American English (AAE). Furthermore, they found that they often cannot demonstrate understanding of what is being said. The authors believe that the different morphosyntax of AAE, the different phonological patterns of AAE, and the different accents in Philadelphia related to residential segregation all conspire to produce transcriptions that not only are inaccurate, but also change the official record of who performed what actions under which circumstances, with potentially dramatic legal repercussions for everyday speakers of AAE.
Jones, Taylor & Kalbfeld, Jessica Rose & Hancock, Ryan & Clark, Robin. "Testifying while black: An experimental study of court reporter accuracy in transcription of African American English." Language, muse.jhu.edu/article/725984.