Got turned away at Singapore immigration because I didn't have proof of onward travel

I booked a one-way ticket to Singapore, planning to "figure out my next destination" once I got there (spontaneous travel, right?). Immigration officer asked for proof of onward travel, a return ticket or a ticket to another country.

I didn't have one. They wouldn't let me in. I had to buy a refundable ticket to Bangkok on the spot using airport WiFi (£200) to prove I was leaving Singapore. Then I had to cancel it later and deal with the refund process.

What I didn't know: some travel insurance policies cover "denied boarding" and emergency ticket purchases if you're refused entry due to documentation issues. Mine didn't, but now I know to check for that.

Has anyone been denied entry to a country? What went wrong? Do you always book return tickets, or do you travel one-way and figure it out as you go?