What happens when you rearend someone and you are excluded from the insurance?

If you were involved in an accident, and it was your fault (generally the person doing the rear-ending is at fault), you are responsible for compensating the injured person for his/her damages. If you have no insurance, then expect to be sued. If you are sued, you need to find an attorney to defend you in the lawsuit. If you do not defend the case, you may wind up with a judgment against you by way of default.

If you are (specifically) excluded from your mom's insurance policy, then that means you were not insured at the time of the accident (unless you have a separate policy on your own car, assuming you have your own car). So, in terms of what you should be prepared for, most would say being held personally liable (*i.e.*, they will sue you). Hope that helps.