The annual report of the CPS inspectorate said errors in calculating how long a defendant could be held in custody had the potential to cause "irreparable harm". In some cases the accused had to be released because lawyers had miscalculated how long he or she could be held, or failed to go to court to get an extension if trial dates changed.
Inspectors uncovered errors in 13 out of the 29 CPS branches investigated.
Chief Inspector Stephen Wooler warned that without improvement, those branches "are and will remain hostages to fortune".
The operational mistakes were just one factor in an overall "pressing need" for the CPS to improve its performance in the Crown Court. The quality of instructions given to barristers employed to present cases in the Crown Court remained a "distinct weakness" and one in four charge sheets contained mistakes.