It's pretty much the same as duff infusion set, your BG would start to rise steadily, you can spot it quite easily as you know your body's reaction to insulin and when something doesn't feel right so a new set change as quickly as possible, i've had a couple of bad infusion set changes, noticed them within 3-6 hours, once i'd ruled out other possibilities I did a change and then took a further 2-3 hours to come right again. Not very scary, just a learning curve