"Please do not leave this page or go back"

UX design leader and instructor, Jared Spool, recently tweeted a common, seemingly-critical warning that we still see too often on eCommerce and bill-payment sites, "Your purchase is in progress. Please do not leave this page or go back."

But why? What will happen if I do leave this page or go back? Should I try to call its bluff or do what it says I shouldn't do out of spite? Some sites will actually try to tell you what will happen if you disobeyed its commands, but when they do, they're often filled with even more system jargon. 

Design for your customers, not for your system's processing constraints

In actuality, I don't really need to know what would happen, because if done right, I should inherently know that I'll need to patiently wait while the system is processing, aka getting me what I want. However, these warning messages are put in place to try to stop us from interrupting their systems with our 'stupid user' actions when their system feedback design fails to satisfy our needs and expectations.

Why would we click this 'Purchase' button again? Maybe because we can't tell that it registered our click or that it began processing. Why would we try to refresh the page? Maybe because the page appears unresponsive or blank. Why would we click back? Maybe because we bought something we didn't want to, or accidentally clicked Purchase before we were ready, and we needed a way to Cancel our order before it's too late.

So when websites and applications aren't adequately designed to satisfy these customer expectations, the systems may get into a critical, disrupted process state when the customer does something they didn't expect to have to mitigate or something they didn't want to cater for with more than a direct, warning message. This is when we get duplicate orders, orders that look like they were successful but failed behind the scenes, page loading errors, and more. 

This got me wondering...

What catastrophic effects would there be if we were given these process-interruption warnings in other moments of our lives and those systems failed to consider our needs?

Your beer is dispensing. Please do not move your pint glass away mid-pour or it'll shatter.
Your elevator is descending. Please do not press another floor button or the elevator cables will snap.
Your shower is warming up. Please do not alter the set temperature or the pipes will burst. 
Your dishwasher is running. Please do not open it or it'll never turn on ever again.
Your friend is thinking about your question. Please do not speak another word or their brain will implode.

Glad these don't follow that same warning - consequence pattern haha.