After passing through St Pancras recently I checked FCC's TVMs to see
if they were fully enabled for Oyster PAYG. For reasons unknown to me
only 2 out of 4 were enabled, though I tried them out anyway.

With the first machine (left side of trio near the Midland Road
entrance), my first attempt involved a £2.80 top up with coins, but
the coin slot was blocked so I cancelled the transaction. The second
attempt involved £5 with a debit card, my PIN was accepted but the
green Oyster light only lasted a second so I couldn't update my Oyster
though the screen stated my card was not charged (which indeed is
true). The third attempt was a repeat of the second except that it was
successful (that said, my current account balance has yet to change).
Hurrah.

Before I could use the second machine which is adjacent to the ticket
office, I had to wait behind someone engaging in a major 'Ticket on
Departure' collection (almost certainly 15 coupons as they call
them).

Once they were done, I tried a £2.05 top-up with coins - for some
reason the chosen value kept changing to £2.04. It wanted exactly that
too but I thought the extra penny wouldn't be an issue - it clearly
was because the coins were all returned. I then tried a £2 top-up with
coins and that worked just fine.