My local station is manned Mon-Sat 6am-1pm. I went there at 5pm on Saturday and the machine was out of order to the extent that it wasn't accepting cards or contactless - cash only. My ticket was £10.70 so I bunged a £20 in the machine. The ticket appeared promptly enough, but no change.
I went to the station this morning and spoke to the guy behind the counter. "Yes, when I emptied the machine yesterday morning, there seemed to be a big surplus in there", said he. He gave me a complaints/comments form (I had already done this online on Saturday whilst on the train to London). I have reclaimed my £9.30, but it crossed my mind that I ought to charge them a penalty of £20 for failing to offer my change on demand.
I wonder how many ticket machines make a surplus like this? I bet that not many people go back afterwards for their change, especially if it is a small amount. Quite often, machines don't display the cheapest available tickets either, so it's quite a scam for the unsuspecting traveling public.
But a lesson learned. If the machine is out of order, don't buy a ticket until you see a human who can sell you one.