The BBC broadcast a businessman's four-letter outburst before the watershed by accident because programme makers could not understand his "strong Bradford accent".
A member of the public complained about the BBC2 show Bradford: City Of Dreams, which was broadcast on 9 May after a scene in which entrepreneur Naveed Khan swore.
In the clip, which went out at around 8.30pm, Mr Khan, who was struggling to understand what one of his colleagues was telling him, was heard to say: "I don't understand f***ing Argentina language."
A report by broadcasting watchdog Ofcom said Mr Khan used the offensive language "under his breath to some extent" but the words were "clearly audible".
The BBC told Ofcom that although the word was "reasonably clear, Mr Khan did say the word under his breath to some extent. This, coupled with Mr Khan's strong Bradford accent, meant that the word was not noted when the programme was reviewed for editorial and compliance purposes".
Two people complained to the BBC who apologised and removed the show from the iPlayer and edited out the language.