The good news is that it’s OK to fall behind – nothing bad happens. Coursera is trying to help you finish the course, not punish you for falling behind. First, each course, except the capstone, only has one hard deadline, and that’s the one for the final project. Still if you miss that, or many quizzes, Coursera may offer to adjust your deadlines, which is to say that it might move you to the next session of the course. You can also do this manually at the bottom of the course info page while logged in (as of this writing), or by going to your course dashboard (follow these instructions). Since these courses currently run monthly, it really just moves you to being on track again with your deadlines.
If you paid for the course, you have a full year to complete it with Coursera – so make sure to complete it within that window, but otherwise there is no penalty in your grades or otherwise for falling behind, for any reason.