Bug in server selection when there are multiple mismatches in the tag sets. E.g., read preference tag_sets like:

[{a: "one", b: "one"}]

And two secondaries with tags:

secondary A: {a: "two", b: "two"}
secondary B: {a: "one", b: "one"}

The C Driver keeps a count of mismatched RS members as it's processing a tag set. Unfortunately, it decrements that count for each mismatched tag instead of each mismatched tag set, so it finishes with a count of zero instead of the correct count of one.