With just days until the European parliamentary elections, Sweden's top candidates for EU seats debated live on newspaper Aftonbladet's web TV channel on Wednesday.
But veteran Social Democrat Marita Ulvskog, a former minister who is now vice-chair of the socialist and progressive group in Brussels, let slip the derisive comment during a moment of particularly heated tension between her and Moderate candidate Gunnar Hökmark
After being cut off by Hökmark, a visibly furious Ulvskog mumbled "f-ing pile of sh*t" ("jävla skithög) under her breath. And her microphone picked it up. When later confronted by the Aftonbladet newspaper, Ulvskog was surprised to learn they'd heard her insult.
"Could you hear that? That wasn't my intention. I didn't realize the microphone could pick that up. It's what I was thinking but I wasn't going to actually say it," she said with a laugh. "That's not how to have a debate."
She blamed Hökmark for presenting facts in a "very twisted way".
The gaffe meant Ulvskog joined a long list of politicians who have thought they were out of earshot when they expressed themselves in less than diplomatic ways. US Vice President Joe Biden used the F word in a moment with the president that he thought was not for public consumption, for example, but his boss could hold the trophy for most politically explosive off-mike-but-not-quite quote.
As then French President Nicolas Sarkozy referred to the Israeli prime minister as "a liar" at a G20 summit, US President Barack Obama famously retorted: "You're fed up with him, but I have to deal with him even more often than you."
Other similar scenarios have involved insults such as "old hag" (about Argentina's Cristina Kirchner) and "clown" (about Italy's Silvio Berlusconi).
Back in Sweden, meanwhile, Ulvskog's opponent Hökmark took the higher ground when confronted about the incident on Thursday.
"A democratic debate is won by truth, openness, and respect for the opponent," he told Aftonbladet.
Swedes head to the polls for the EU elections this weekend.