If a word's spelling was standardized prior to sound changes that produced its "traditional" pronunciation, a spelling pronunciation may reflect an even older pronunciation. This is often the case with compound words (e.g. waistcoat, cupboard, forehead). It is also the case for many words with silent letters (e.g. often), though not all—silent letters are sometimes added for etymological reasons, to reflect a word's spelling in its language of origin (e.g. victual, rhyming with little but derived from Late Latin victualia). Some silent letters were added on the basis of erroneous etymologies, as in the cases of the words island and scythe.
Spelling pronunciations are generally considered incorrect next to the traditionally accepted, and usually more widespread, pronunciation. If a spelling pronunciation persists and becomes more common, it may eventually join the existing form as equally acceptable (for example waistcoat and often), or even become the dominant pronunciation (as with forehead and falcon). If a rare word is more often encountered in writing than in speech, the spelling pronunciation may be assumed by most, while the traditional pronunciation is maintained only by older or educated individuals.
Large numbers of easily noticeable spelling pronunciations only occur in languages such as French and English where spelling tends to not indicate the current pronunciation. Because in all languages, even in languages such as Finnish, at least some words are not spelled as pronounced, spelling pronunciations can arise in any language when the majority of the populace only obtains enough education to learn how to read and write, but not enough to understand when spelling doesn't indicate modern pronunciation; in other words, when many people do not clearly understand the relationship between spelling and pronunciation.
On the other hand, spelling pronunciations are also evidence of the reciprocal effects of spoken and written language on each other. Many spellings represent older forms and corresponding older pronunciations. Some spellings, however, are not etymologically correct.
Though many people may believe (to various degrees of accuracy) that the written language is "more correct", this (in turn) can become a self-fulfilling prophecy, with the written language affecting and changing the spoken language and resulting in either a pronunciation similar to an older one or a new pronunciation, when a spelling does not represent an older pronunciation.