Whenever venting the roof there could be obstacles that you can’t see upon arrival. 
The pictures shown are just two some of the things you can encounter. Double roofs are unfortunately extremely common and truly unexpected. Also known as a rain roof, these may be found on commercial structures. You need to always keep this in the back of your mind. The second picture was taken right after the firefighter finished his cut of this roof. 
All of that for lack of a better word, junk, jammed in the saw. This could happen in the middle of the cut which would cause a major problem while trying to vent. Always pay attention to construction in your first due and mutual aid jurisdictions. While some of these elements aren’t avoidable, having the knowledge to overcome them is what matters.
Flow and Vent 