I'm wondering if "make sure" can take the subjunctive mood of a verb. Take this sentence for example:
Loki would be disappointed in me if I did not make sure this was the true casket.
What if I said:
Loki would be disappointed in me if I did not make sure this were the true casket.
There surely is a feeling of uncertainty, it could, hypothetically, not be the real casket, that's why he has to "make sure", although I'll be the first one to admit it sounds weird, but most constructions with the subjunctive sound weird.