What if you violate your felony probation by dropping a dirty ua?

They may let the first test past with a warning but do not have to.

Normally you will be violated, picked up and put in a jail or probation center pending a court hearing.
At the hearing the judge will decide if he will send you to jail for a short time, or prison to serve your entire sentence.
The judge may also merely warn you and put you back on probation. May also require a program directed at the problem of the dirty UA

You will be violated, held in jail until you have a VOP hearing. The judge may decide to revert your probation and sentence you to the original sentence instead of giving you more probatgion or just a warning. Go back and reread your original sentencing paperwork and see what the judge had in mind when he first sentenced you. He will be consulting with that same paperwork when you come before him for the VOP hearing.