This has happened a couple of times with Singapore but not other airlines. It seems that if you select your flights and run through the booking process, even if you don't pay it reserves your flights on the system ...even without payment. I left the payment page as my laptop battery was very low, so presumed it'd time out and then I'd book later after searching for the flights again.

Is this just a flaw in the Singapore website or does this happen with other airlines?

I had to speak to support and book over the phone to finally make sure the reservation wasn't paid for.