Common exception words are words in which the English Spelling code works in an unusual or uncommon way. They are not words for which phonics 'doesn't work', but they may be exceptions to spelling rules, or words which use a particular combination of letters to represent sound patterns in a rare or unique way.
Common exception words are words that do not follow the common phonetic spelling rules children learn in Year 1 and Year 2. Many of these exception words are used frequently, hence the use of 'common' in the name. Children are normally introduced to common exception words in their first year of school.
e.g. because, hour, grass