You are correct that this is a despensing error and it can have very bad and dangerous implications. This is fault from the pharmacy side and they are responsible for this.
This type of medications are very critical as to achieve the desired effect, it should be given in proper dose and if the dose is not proper than its therapeutic benefit can not be achieved. Another important thing is that you have to do the blood investigation to know if the medication is working and your bloos clotting status is what it is required to prevent the clot formation.
I think when you have your blood test
than you must have consulted your doctor again with the test and if there was a very significant finding than it must be pointed out.
It is good that you got to find out this and now onward be more careful when getting some medication in pharmacy and cross check that with your doctor also.
Hope this will help. Thanks