Hundreds of US election adverts were wiped out on Wednesday by an apparent Facebook glitch that left some campaigns crippled in the final stretch of the contest.
Republicans and Democrats had scrambled over the weekend to meet the social network's deadline for a seven-day freeze on new political adverts, which began on Tuesday.
But many submitted their adverts in time only to see them suddenly removed by the company, while one campaigner reported error messages when trying to upload adverts on Monday night.
Meanwhile, adverts by President Donald Trump that broke Facebook's rules against false voting information were approved and seen as many as 731,000 times before being taken down.
That prompted anger from Democratic campaigners, who said that any unplanned downtime would cause "irreparable damage" in the crucial seven days before polls close on November 3. Some advertisers reported still having problems on Tuesday night.
Rob Leathern, Facebook's advertising enforcement chief, said: "We're investigating the issues of some ads being paused incorrectly, and some advertisers having trouble making changes to their campaigns.
"We're working quickly on these fixes, and will share an update once they are resolved."
Facebook first announced its advertising freeze in September after months of pressure to restrict, fact-check or entirely ban political advertising.
The new rules, intended to ensure that journalists and the public have time to scrutinise any major messaging blitz, allows advertisers to change the targeting and spending of adverts that have already been seen at once before the deadline.
That has led campaigns on all sides to upload all their adverts for the last week of the election in advance, ready to push them into voters' feeds later on.
Yet the outage, first reported by Protocol and AdWeek, left many such plans in disarray. According to AdWeek, both the Biden and Trump campaigns were affected.
Josh Schwerin, communications director at the Left-wing fundraising committee Priorities USA, said 586 of its adverts had been incorrectly rejected, despite receiving impressions before the deadline.
"With seven days left, every hour is critical. Even if this gets fixed soon the damage is irreparable," he tweeted, later adding: "This has not been fixed. Facebook has cost us an entire day with these ads."
Rob Flaherty, head of digital campaigning for Mr Trump's opponent Joe Biden, lambasted the freeze as a "silly, performative hoop-jumping exercise", and the outage as its predictable result.
Max Rymer, a Republican advertising consultant in Minnesota and director of the digital marketing agency Nativ3, said he had also had errors, and described the entire situation more succinctly as a "total s--show".
At the same time, Facebook did not initially take action on a set of adverts from Mr Trump that urged viewers to "vote today", and in some cases falsely claimed that "election day is today".
Facebook had told campaigns in advance that it would ban instruction to "vote today" without any other context. The adverts also appeared to have been approved after the freeze deadline had passed, and were seen hundreds of thousands of times, largely in swing states.
Julia Rosen, a partner at the Left-leaning Fireside Campaigns, said: "We carefully followed all of Facebook's rules.. after we were explicitly told not [to] do date-specific ads, Trump's team decided to just do what it wanted, assuming Facebook wouldn't dare stop them."
A spokesman for Facebook said: “As we made clear in our public communications and directly to campaigns, we prohibit ads that say ‘Vote Today’ without additional context or clarity."