Ok, if I've got this correct...

With Oyster Pre-Pay, if you don't swipe out when you leave the network
(for example, if the gates are open already) then you get charged £3.80
- no matter how far you go. So, if you only got 1 stop and forget to
swipe you get penalised.

However, yesterday I went all the way to Amersham and for this journey a
single fare is a whopping £5.50!

Unfortunately, the gates were closed so I had to swipe out when I left,
but if they were open (or if the side exit was open) presumably I
could've saved myself a bit a money by accidentally 'forgetting' to
swipe out.

Do they automatically stick the £3.80 charge onto your card though?
I've heard of a few people who have had to go to the ticket office
becuase of unresolved journeys - if you get one of these do they
interview you to find out where you were going etc.?