A woman received a warning about imminent flooding - nearly an hour after the ground floor of her home had been left under water.
Anthony Osbourne, whose 89-year-old mother Patricia lives in Nantgarw, said she had received no alert before.
At about 04:30 GMT on Sunday, water from the River Taff flooded into her home after Storm Dennis.
As she battled to deal with it, a warning from Natural Resources Wales (NRW) of "imminent flooding" arrived.
This was at 05:20, 50 minutes after water had entered her home.
Mr Osbourne said on Tuesday an assessment had been made of his mother's home, and it was lucky she had insurance for the family home.
"She is still a bit shell-shocked, she keeps thinking she is going back home," said Mr Osborne, who said his mother remembered major floods in 1960s.
How difficult was it warn everybody?
To put the scale of reaching everybody into perspective, during Storm Callum in October 2018, 50 flood alerts and 40 flood warnings were issued over four days.
During a three-hour period at the height of Storm Dennis, between 09:00 and 12:00 on Sunday, there were 61 flood alerts, 86 flood warnings and two severe flood warnings in force.
Who was warned?
There are about 126,000 homes across Wales whose owners or tenants are signed up to warning alerts from NRW.
Messages were sent out over the weekend to 26,333 people in 11,676 properties.
Warnings were sent in a variety of ways, with 75,551 telephone messages, 16,953 texts and 18,685 emails.
People had chosen beforehand in what form their messages were sent.
A spokesman for NRW said people in Pontypridd were sent a number of messages as conditions changed.
He said there could be a number of reasons why messages are not received - such as a phone being engaged or the message not being retrieved from an answer machine.
What has been learnt?
"Following storms and flooding of this magnitude, we carry out reviews into the performance of our systems, defences and modelling," the spokesman added.
"We have learnt a lot from previous reviews, which has resulted in improvements to our flood defence and flood warning capabilities."
He said the immediate priority was working with local authorities and police to help affected communities.
"Our staff are working to check our defences for damage, clear assets of debris and continue to monitor river levels," he added.