Error not pronounced — it's easily undone (4,4)
I believe the answer is:
slip knot
'it's easily undone' is the definition.
(I know that a slip knot is easily undone)
'error not pronounced' is the wordplay.
'error' becomes 'slip' (slip is a kind of error).
'pronounced' indicates a 'sounds like' (homophone) clue.
'not' is a homophone of 'knot'.
'slip'+'knot'='SLIP-KNOT'
(Other definitions for slip knot that I've seen before include "that's easily undone" , "Something tied but easily undone" , "Granny, say" , "You can see it run along rope" .)