Certain paths with certain Offset Path values create these.
Usually it’s a rather small value, but this sometimes happens with larger ones.

There are other related problems with Offset Path:
1. Stroke displaying incorrectly and Offset Path broken — http://illustrator.uservoice.com/forums/601447/suggestions/39059872
2. Offset Path with small values (or Inside / Outside strokes) results in skewed lines — http://illustrator.uservoice.com/forums/601447/suggestions/39425959