If a post begins from a smile, the this smile will not be parsed in the quote. If you add a space before the smile code - it parses.

1) BAD:

user:

:mrgreen: text

without BB code it's:

{quote="user"}:mrgreen: text{/quote}
you see ":mrgreen:" instead of smile's image

2) GOOD:

user:

:mrgreen: text

without BB code it's:{quote="user"}

:mrgreen: text

{/quote}

space added, and you see smile's image