'I tweeted to ask if this was a joke ... they said unfortunately not'
Among those angry at the error was 27-year-old Klara Rebecca Leigh from Holywood, Co Down, who said she was shocked after realising an email sent by the charity was "not a joke".
"I checked my emails on Friday night and saw an email from Rose Caldwell, who is the director of the charity. I think it was a generic email saying that they are really sorry this has happened, and that I was going to be debited £500 instead of £5 from my usual monthly donation.
"It then said any charges incurred would be refunded. There were really no other details.
"I then thought it could be spam. I rang my bank immediately but I hadn't heard anything about this in the press, or broadcast media. I told the bank and they said there is nothing they could do but contact them as soon as it goes through and they will deal with it.
"I had tweeted Concern and asked 'is this a joke?' They got back to me and said unfortunately not.
"I just dealt with my bank Nationwide and they have been fantastic.
"I went back in on Saturday morning as I was so worried and the girl was shocked but put a note on my account that as soon as it happens they have put a block on it and will call me.
"I've now cancelled my direct debit and that block from the bank will mean that it can't be reinstated.
"There are people who could have a credit warning. I won't support Concern again, through direct debit at least.
"It is not what this will do for Concern, but for charities in general through online donations.
"How something like that can happen in this day and age, I just don't know."