Some embodiments of an infusion pump system include a pump device with a drive system that is configured to attach with a medicine cartridge in manner that reduces the likelihood of dosage inaccuracies. In one example, the drive system of the pump device can be equipped with a plunger engagement device that is configured to mechanically anchor into a plunger of the medicine cartridge, and then to bias the plunger of the medicine cartridge in a direction toward a piston rod of the drive system.