Bring back ticket offices!
TVMs are pretty poor (bad software, limited ticketing facilities - although some TOCs appear to be trying to improve them) but I think that with the combination of poorly trained, or downright lazy, staff and agency staff that don't really care about the damage they may do, the days of the manned ticket office are numbered.
Besides asking for very simple tickets (one day travelcard, return to London etc), I regularly find myself struggling to get what I want. I've had staff refuse to sell a ticket I got only a few weeks before, or get told that a ticket doesn't exist - until I go onto my phone and show a photo of the ticket I had and they then sigh, and ultimately find the ticket does exist and make up some silly excuse.
Everytime I travel now and have to ask for ticket that is a tiny bit out of the ordinary, I feel like I'm taking a gamble and always wish I'd gone online and bought the ticket(s) there, which of course is another reason ticket staff will soon become obsolete.
I totally agree about getting some evidence though when something goes wrong. Photograph the TVM(s) saying 'out of service' if you can, although you might still be told it could have been taken at any time!!