I know that very stringent checks are in place in the labs in the teaching hospitals attached to my university, although I'm not 100% positive the procedures are exactly the same as the last time I took an interest in them.
As I recall, the samples must be accompanied by the usual request form with the patient's surname, forenames, dob, gender, patient number, ward identifier, reason for transfusion, diagnosis etc fully completed.
If there was a discrepancy on the form, the sample and document would be returned by the MLSO's/blood bank staff to the originator, unprocessed for correction. If I remember rightly, this would happen no matter if the request was classified as urgent - departmental pressure was not an excuse.
It was vital that the blood bank verified the patient's ABO and Rh D group via their computer database/manual records and any discrepancies were resolved before blood or blood components were issued, although in life threatening situations, group O Rh D negative or positive blood was issued depending on the gender of the patient.
As a double check it was also necessary to mix a small sample of the patient's blood with the donor blood and check for signs af agglutination under the microscope. Obviously, in the event of agglutination, there was a mismatch. Specialised sampling equipment was used to perform this task in a minimum time.