The word mischievous is sometimes pronounced with a long e sound between the v and the last vowel (mis-chee-vee-us), although this is controversial.
Is there a name for this type of word, where the pronunciation is made more complex than the spelling indicates?
The name for the type of error is misconstruction:
misconstruction 2. Grammar A faulty construction, especially of a sentence or clause.
Although the definition refers specifically to sentences or clauses, it can be used for single words as well.
From NOAD, for those who think “mischievious” is a variant:
usage: Mischievous is a three-syllable word. Take care not to use this incorrect four-syllable pronunciation: |misˈCHēvēəs|.