Position of 'dernier'

Dernier is one of the adjectives that changes meaning with positioning before (final) or after (previous) the noun. Usage with «fois» is a special case where it can be placed before or after, if the phrase 'la dernière fois' or 'la fois dernière' is not followed by a clause. However, as «la dernière fois» is always correct, with or without a clause following, it is perhaps prudent to use this mostly. Dernier = final/previous (French Adjectives that change meaning according to position)