Fewer than half of pupils who left primary school last summer having reached the expected level in English knew how to spell "knowledge".
Common mistakes included "nowledge" or "noledge", said a report on 11-year-olds' performance in the 20-word national spelling test taken in May.
There were six words which the majority of pupils who reached level 4 could not spell.
As well as "knowledge", the pupils stumbled over the double f in effortless, suggested "partisipate" for participate and heard a "c" in disguised. Rehearsed and thoroughly were the two words which also taxed the most able children.
The Qualifications and Curriculum Authority, which analysed the results of 215 pupils, said the children relied too much on sound to predict some spellings, rather than recognising that the root of the word knowledge is know.
Its report on standards at key stage 2 says: "Some of the more complex words in the test, such as thoroughly and rehearsed, have spellings which need to be visualised and memorised."
The English test also asked pupils to write a short radio advert and a longer story based on four pictures of a boy queuing outside a toy shop.
Pupils falling short of the expected level 4 were able to tell a simple story about the queue, but more than half struggled with punctuation.
The highest achieving children were praised for their character development and for building up their stories to a dramatic point.
Chris Davis, of the National Primary Headteachers' Association and head of Queniborough primary, Leicestershire, said: "We found the new tests favoured the way we teach, focusing on children who think for themselves and write their own thoughts rather than by formula.
"Our writing scores went up dramatically."
The QCA report also looked at how pupils fared in the maths and science tests.
It includes advice to teachers to ensure children have direct experience of compost heaps and rotting fruit after four out of five 11-year-olds who reached the expected level in science were unable to think of any way in which micro-organisms were useful. They were told not to use the example of preventing or curing diseases.
At KS 1, teachers have been told to urge seven-year-olds to read maths questions carefully before trying to answer them.
A common error was for children to ignore subtraction signs and add up any figures they were given so "30 - 15" became 45 rather than 15.
Year 6 children found it tricky to work out numbers which fitted several criteria during their maths tests. They were asked to calculate k, m and n which added together to make 1,500, where m was three times as big as n and k twice as big as n. The most common error was to give values that added up to 1,500 but ignored the second part of the problem about the proportions.
Primary forum 24
WHERE THEY WENT WRONG ...
Spelling mistakes that more than half of seven-year-olds made in their key stage 1 tests:
* now (instead of know)
Spelling mistakes that more than half of 11-year-olds made:
Slips in the key stage 2 science test:
* Almost a half of children working at level 4 did not correctly describe the earth as a sphere, instead saying it was round or a circle.
* Given a diagram of the Earth with clouds above, below and on either side, most children correctly drew in arrows to show the rain falling on earth.
But some thought that rain may fall "down" to the bottom of the page.
* Given half a graph showing how shadows shorten from the morning to noon, only a third of children at level 4 correctly completed it to show the shadows lengthening again in the afternoon. Two-fifths of children continued the graph's downwards line, and a few drew in zig zags.
* In a "novel and challenging" question, 11-year-olds had to suggest something better than a tape measure for measuring solid balls. Less than half of children said they could be weighed. Many others suggested a ruler.