During the ten letter round, the sequence of vowels and consonants pulled by Riley initially spelt out an innocuous jumble, “IDLORUEJZ”.
But once the clock had run down, a contestant announced his winning word, “LOUDER”, which left the maths whizz quickly rearranging the tiles - and was left with “JIZ”.
As the camera panned back to Riley, the board now read: "JIZ LOUDER."
After clocking the awkward coincidence, the Channel 4 star, 36, couldn’t help but smirk as the inappropriate phrase appeared on the board for an extended amount of time.
Despite showing her professionalism, viewers were anything but as they flocked to Twitter in their droves to share their thoughts.
Applauding Riley’s composure, one fan commented: "JIZ LOUDER!!! If I was Rachel I’d be on the floor rolling around with laughter. Fair play to her."
"I bet it’s killing Rachel inside not to laugh or comment on this," another Countdown viewer remarked.
A third tweeted: "Rachel Riley is a master of butter wouldn’t melt mischief."
"OMG! Countdown never disappoints. Jiz Louder hahaha," another wrote about the hilarity that had unfolded on daytime TV.
Although the former Strictly star was able to hold back the laughter this time, the Oxford graduate has previously burst into laughter when the board has spelled out some rather questionable phrases.
Earlier this year, the presenter was left giggling after one of the contestants came up with the phrase “Up vaginas”.
After seeing the board, Riley turned to her and quipped: "We love vaginas don’t we Jo?" to which the funny woman quipped: “Oh we love vaginas!”