According to the Gambling Commission, which fined bookmaker William Hill £6.2m for "systemic failures" on Tuesday, one customer was allowed to deposit £654,000 over nine months without any check on their income, which was around £30,000 per year at the time.
Another customer was identified by William Hill as having an "escalating gambling spend", but was allowed to continue gambling after the bookmaker received an "assurance" that they were "comfortable" with their spending.
A third customer, who was also making around £30,000 each year, was allowed to deposit £541,000 over 14 months after a William Hill official assumed his income could be as high as £365,000.
This assumption, the Gambling Commission said, was based only on a "verbal conversation" had with the customer, with no further questions asked.
In reality, the customer was funding his habit by stealing from his employer, the commission said.
Over recent years, many former gambling addicts have spoken about their broader experiences with gambling, with many pointing to fixed-odds betting terminals (FOBTs) as a particular danger.
Matt Zarb-Cousin, a former spokesman for Labour leader Jeremy Corbyn who now campaigns for fairer gambling rules, has written about how FOBTs almost led him to suicide.
He wrote in the i newspaper last year that he first became addicted to the machines as a teenager on his lunch breaks from school.
The betting shop did not ask him to prove his age, he wrote.
He found a job in a call centre, but said most of his income went straight towards his gambling habit.
He wrote in the paper: "It is the unusually high staking capacity of FOBTs that facilitates the addicting roulette content, which is played at a speed of one spin every 20 seconds."
His addiction continued while he was a student at the University of Birmingham, he wrote.
"It dawned on me that the implications of this were that I may have to quit university.
"With no access to money, no ability to escape the problems gambling had created by - perversely - gambling, and with only reality to face up to, I became suicidal.
"My losses by that point totalled around £16,000 and I was so messed up that I saw no way out other than to take my own life."
After an intervention from his family, he was able to stop gambling at the age of 20.
He now campaigns to reduce the maximum FOBT stake from £100 to £2.
Jim Shannon, a DUP MP, said in Parliament in 2016 that he knew of two suicides linked to FOBTs.
He said there was "no place" for £100 spend games in high street bookmakers, with "little or no supervision", adding: "The regulation of FOBTs is out of kilter with the principles of gambling regulation."
MPs have also warned about the dangers of criminals using the machines for money laundering.
Labour MP Carolyn Harris, chairwoman of the All Party Parliamentary Group on FOBTs, has described the machines as a "heaven for money launderers".
She said £2 million was lost in one year in her Swansea East constituency to the machines, with 20 betting shops licensed to provide activity in the area.
This equates to £25,000 lost on each machine, she said.