Never get between a determined shopper and the checkout. Customers clutching free-money vouchers may go off their trolleys when that free money is snatched away.
The ACT government knows that now. It's "paused" the ChooseCBR scheme after a crash.
We may look at the unfortunate event in two ways.
Let's be charitable first. The plan to pump up spending was so well targeted that the demand was overwhelming, not least of the technology. This was a triumph of conception but not of execution.
Or, a crash is a crash is a crash - and an online crash ought to lead to some sort of political crash.
Sad to say, this may be the more popular view in our social media age where blame is the aim of those who presumably never, ever make mistakes in their own lives. They click and clack and call for heads to roll.
But you can understand the frustration. Shoppers went to their phones and downloaded the free-money vouchers offering slashed prices. They rushed to the store - to find the bargains snatched away at that last climactic moment.
Imagine the scene at the "adult stores" included in the scheme. With punters panting in anticipation of the imminent purchase, the gremlins deep in the internet intervened. You can almost hear their thunderous laughter.
The poor shop assistant was caught between the frustrated customer and the stupid machine.
It's no wonder that Deakin IGA, for example, put a sign in the window.
"The system is unreliable and causes too much distress for our staff and customers," it said.
"We apologise for trying to participate in this program. Please understand this is out of our control."
So who's control is it in then?
Nobody's, really. That's the problem with this marvelous technology which has transformed our lives. For most of the time, it is a fabulous boon - but when it goes wrong, it goes very wrong.
But we should resist the urge to blame. No heads should roll. Keep calm. Those in the adult store should take a shower. Cool down and have a cup of tea.
This was the second version of the scheme. It was given an underwhelming trial back in December, and only $310,000 of the available $500,000 was spent over three weeks. Take-up wasn't very high.
But with $140,000 spent on the first day of the revamped scheme, demand was high for the taxpayer-funded vouchers offering discounts of up to half. That indicates the scheme is popular.
The system has won, at least for the moment.
The minister for business Tara Cheyne said: "We are very conscious of the inconvenience and frustration this has caused to both businesses and customers, despite our best efforts to fix the problems to date."
Let the machine have its victory - but clever humans are working hard to fight back. Public servants will serve the public and defeat the glitch - or so we hope.
There are worse things to worry about - like who won the unclaimed $4.8 million in the lotto. Was it you?
Free lotto tickets would be a thought but maybe get the software right first.