If they can be nailed wrong, they will be. It seems that more than half of the decks we inspect have a problem with the joist hangers. For some reason (and its probably price) people want to buy a smaller joist hanger than necessary saving a few dollars and putting the deck in jeopardy.
Joist hangers must match the size of the joist it is to support, so a 2×8″ joist needs a larger joist hanger than a 2×6″ joist.
Then it must be attached using special approved nails, which means that finish nails, drywall screws, framing nails and least of all and most often used “roofing nails” and not approved. Roofing nails will fail because if they are put under a load the heads will break off.
The approved nails are stubby galvanized and specifically made for securing joist hangers. Equally there must be a nail in every nail hole to meet the manufacturers requirements.