Mylan Labs makes an Epipen for adults and an Epipen, Jr. for children. They do not make an Epipen for cardiac use. Epinephrine for cardiac use comes in a prefilled syringe with an 18 gauge needle and could never be mistaken for an Epipen.
When Linda is talking to Jamie about the Epi Pen being the wrong dosage, she mentions that it was filled with Cardiac Epinephrine implying that it was a larger dose. Epi Pens are pre-filled with Epinephrine in a concentration of 1:1000. Cardiac Epinephrine has a concentration of 1:10,000. The pre-filled vial in an Epi Pen would therefore contain LESS epinephrine due to the concentration being lower, thus a lower dosage administered. Furthermore, Cardiac Epi used on Medic Units come packaged in an ampule that must first be assembled and is designed to be used with an IV Injection port on the IV Line (the Y port below the roller valve that controls IV drip rate).