My daughter had a near-experience with this. Typically, you enter two values into the pump: your current blood glucose level, and the number of carbs you are going to (or in the case of a child, have) eat(en). The pump uses a conversion formula to dose you for the carbs, either shorting the amount if your glucose is low, or increasing the amount if your glucose is high.
A school nurse new to the pump reversed the numbers. My daughter had eaten about 60g of carbs, and her blood glucose level was ~160, but the nurse entered blood glucose of 60, and 160g of carbs--something like 6 units of insulin. Little girl had to stuff chocolate and apple juice into her face all afternoon, which doesn't sound bad unless your blood sugar's on the floor and you're sick to your stomach. Never thought we'd have to beg a child to eat more chocolate.
One lesson (among many) I took from that is that drawing fluid into a syringe is intuitively obvious whether it's a common-sense amount or not (for one, her old syringes wouldn't even hold 6 units), whereas two numbers on a monochrome screen are not.