Salesforce Platform Developer I (CRT-450) Test 1

A lead object has a custom field Prior_Email__c. The following trigger is intended to copy the current Email into the Prior_Email__c field any time the Email field ischanged:
trigger test on Load(before update){
  for(Lead Id: trigger.now)
  {
    if(Id.Email != trigger.oldMap.get (Id.Id).email)
    {
      Id.Prior_Email__c = trigger.old, ap.get(Id.Id).email;
      update Id;   
    }
  }
}
Which type of exception will this trigger cause?