Kelly Haynes, from Islington, north London, bought learning aids from her local branch of 99p Stores, which contained a schoolboy howler on the 12 times table.
A schoolboy who perfectly memorised his multiplication tables still managed to get one less than top marks - because the discount store poster he'd been using to learn got its 12 times table wrong.
Kelly Haynes was livid when her eight-year-old son came home from school 'distraught' because he'd been marked down on his maths.
The dud poster claimed 12 x 12 was 148, not the correct 144.
Supermarket worker Kelly said: 'My eight-year-old is starting to do proper times tables in school so I thought I'd help him out with a treat.
'He had a test one Friday and came back really upset and said "I got one wrong I put 148 for 12 x 12 and they said it was a mistake but it's not."
'I said "the answer is 144 honey" and he went "no it's not, my poster says it's 148."
'When we got home I saw he was right, and he was still so upset and demanded I take them down because he would have got 100 per cent in his test if there wasn't this huge mistake.'
After removing the poster, furious Kelly stormed back to the shop to give the selling staff a bit of her mind.