I’ve just experienced what is possibly the single worst piece of user experience I’ve had to interact with.

I go to top up my Oyster card. There are two machines. One has a long line in front of it, the other is empty because it’s cash only due to a broken card reader. I have plenty of cash on me, so I decide to skip the queue.

This, it turns out, is a mistake.

So first it asks me how much I want to top up. My options are helpful prefilled buttons of 10 through 50 or I can enter an amount. I try to enter an amount larger than 50, but it helpfully informs me that despite the ever growing costs of public transport in London it’s literally inconceivable that I could want more than £50 on my Oyster, so I’m not allowed to do that. Fine, I ask for £50. It tells me to give it the money.

I put in two £20s and hunt around for a £10, but I turn out to only have £20s (For some reason it does not occur to me until later that I could have just put in an additional £20 and got change). Can I just ask it to accept that? No. Oh well. I click “Go back” assuming it will keep my money and let me choose another option.

Nope. Apparently “Go back” means “Abort everything, go back to the very beginning”. Oh well.

But as I press go back and am returned to the beginning of this sad saga, there is an ominous “clunk” noise.

Which is then followed by the distinctive metallic rain sound of a shower of coins onto hard plastic as my nice, compact, £20 notes are converted into individual £1 coins.

“Oh you’re fucking kidding me” I exclaim, as I stare at the machine in disbelief. The lady next to me at the other machine gives me an amused smile. “Ha ha, sucker” she carefully refrains from saying. “Should have waited in the queue”. She completes her transaction and moves on.

Meanwhile I’m left with a giant pile of pound coins and an untopped up Oyster card.

I sigh dramatically and begin to feed the coins into the machine. You know those nice big coin hoppers they have at a lot of self checkout machines where you can just pour your coins into it? Oyster card machines don’t have those.

I gather up handfuls of coins at a time and start individually feeding them into the tiny coin slot. Clunk. Clunk. Clunk.

After £27 it decide it’s had enough and just starts spitting out any more coins I feed it. I can’t really say I blame it. I mean who the hell would want to have that many pound coins, right?

I press the go back button again. Metallic rain.

At this point, the machine has defeated me. I gather up my coins in two great big handfuls, cross the hall to the empty ticket hall and talk to the nice man behind the desk.

He looks bemused as I pass him my pile of coins, but is perfectly happy to count them up and put the money on my Oyster card. The entire transaction takes about two minutes.

The annoying thing is that as a programmer I know exactly the sort of dysfunctional processes that lead to this sort of thing. But, as a user, I sure wish they’d actually watched some people trying to use these machines with cash before they shipped them.