It's strange - when commuting to school in the morning on a 3-car 159 (it would have been the 0739, now the 0744, to Salisbury, now to Gillingham (Dorset)), a few friends and I always sat in the rear vestibule (almost always first class). We didn't realise at the time it required a first class ticket (I wasn't a rail enthusiast until much later), and we were never challenged by the guards - in fact, the closest thing we had was a single guard (a single guard in around two years worth of travelling every school day!) telling us with a grin on his face that it was first class and so we shouldn't be there (but presumably in jest since he didn't charge us extra or say anything else and immediately moved on down the train...).
Very few people sat in first class in general, and even fewer of them got out of the rear doors (perhaps they're scared off by the emergency exit sign), so it's not like we were blocking anyone (it's also further from the exit than the doors further forwards).
We only stopped doing it when the train became less busy and we could usually get a table seat - as I said, we were never challenged except that one time which we assumed was in jest. And as far as I can tell, unlike the habitual behaviour mentioned in this thread, it never harmed or delayed anyone.
I wouldn't do it now that I know.
