While looking at a patient in CMS 61v4, I noticed that a count cause was evaluating as true when it shouldn't for the denominator evaluation. The clause needed 2 of the criteria to be met. There was only one criteria which met the requirements of the measure, however Bonnie was insisting that there were two.

Upon looking closer inspection I learned it was because there were two patient characteristics with different codes that fit what Bonnie was looking for. Normally this wouldn't be a problem, but they were coded as happening at the same time. Thus, although Bonnie was supposed to fetch one instance, it fetched two instances, which allowed the patient to fall into the denominator.

I have attached screenshots of a mocked up patient to demonstrate this issue.