Vodafone has apologised to its customers after a "technical issue" led to some people being wrongly billed thousands of euros in roaming charges.
The phone company said in a statement on Monday that it was "really sorry" for the error but that "it may take some time" to fix.
"You will not be charged for any incorrect amounts," it added.
But despite the apology, a number of Vodafone customers have shared their panic on social media after discovering bills that racked up to more than €11,000, and finding their connection had, as a result, been cut.
UK-based user David Gordon, who is on his honeymoon, said the discovery of a £9,567 (€10,000) bill had made his trip "very stressful".
He added: "To say it put a dampener on our honeymoon is an understatement."
Another holidaymaker in Malta said he had been charged nearly £5,000 (€5,600) for data over one night, and that his network service had also been cut.
"I've been here a week and used probably 10gig without a problem. Tried calling Vodafone this afternoon and after an hour my network had been cut off!
"So can't even contact you. HELP."
Many other holidaymakers across Europe — which is part of Vodafone's "roam-free destinations" — also reported issues, with some saying the company error had "ruined our holiday".
One person, speaking on behalf of his father, said: "Probably wouldn't have been as much of a shock to him if [Vodafone] had sent out a message telling folk there was a fault."
Meanwhile, some customers began tweeting to find out who had received the highest charge, with Southampton user Michelle asking if anyone could beat her bill for £10,127 (€11,500).
In an updated statement on Monday afternoon, Vodafone said it understood the "frustrations" of its customers, adding that it would "proactively be in touch" with those affected to discuss compensation.