LM has no facitility for checking Oyster Cards, so it wouldn't surprise me if enterprising souls are dumb-belling between Watford, Bushey, Harrow & Wealdstone and Euston.
I think you'll find they'll also be using the Oyster cards as far as Kings Langley, Apsley, and I've even had one silly cow tell me that "it's okay, my Oyster card works as far as Hemel, innit......"
The classic trick for the "Oystered" commuters that get on at the above stations is to be in the middle of the train at WFJ - there's a reader by the station building, so they leap off, tap in, and leap back on. They know they don't have a huge chance of getting checked at that time of day because the train is
a) normally a 12 car, and you MUST be at cab 2/3 at Berkhamsted for the despatch monitor - which they know!!
b) so busy that they know the chances of you coming through are slim to nil.
c) normally booked to run into platform 18 at Euston, because it goes out empty to Camden, and of course those of you in the know will know that there are no barriers at that end of the station!!
That explains why so many of them looked soooooo pained on Friday when I came through all 12 cars because someone was working the doors for me!!!!!
Some of them actually looked like they'd trodden in something unpleasant when they saw me coming, and one or two even commented that they'd "not been checked on this service for months".