Non-cartridge pens have a 3ml cartridge in them, it just can't be removed.
If something goes wrong with the injection mechanism in the pen, a syringe can be inserted in to the rubber needle access and insulin drawn, just like from a vial.
A few syringes should be in every insulin-using diabetic's kit, for emergency situations. Also, if you're a fan of intramuscular injections for faster corrections as I am, keep some long needle ones around (12mm).