Answer
Parsimony can give inaccurate results if there were multiple or contradictory steps in an evolutionary event that cannot easily be captured. For example, if a reversion occurs-- this makes it seem as if no change occurred where two (one change, one reversion) occurred. This can lead to organisms being placed back into the ancestral group when they should be in a new one.
Work Step by Step
This question is showing you that the principle of parsimony (see the text where it discusses Occam's razor), though useful in phylogenetics, can be wrong. It cannot be used blindly.
