The craze has seen soaring numbers of women posting pictures of themselves wearing no make-up on social media sites.
Cancer Research UK, which did not start the campaign, said the #nomakeupselfie trend had resulted in an "unprecedented increase" in donations after it received £8m in six days.
But it has emerged that some people sent their cash to Unicef in error, while others had mistakenly enquired about adopting a polar bear with the World Wildlife Fund.
Cancer Research UK and Unicef use the same text number for donations but those texting the keyword "DONATE" rather than "BEAT" found their money sent to the UN agency.
Mike Flynn, director of individual giving at Unicef UK, said it was working with Cancer Research UK to transfer £18,625 which was donated by mistake.