US 1513 (Vfw) - Perforation Shift

Quote:
Error - a major and total production mistake, often defined as a mistake that can be identically repeated. Examples: Inverts (such as Scott C3a), imperforate stamps that were intended to be perforated, use of the wrong perforation gauge, and totally missing colors or tagging.

Freak - a mistake in production caused by an unusual circumstance not likely to be identically repeated, such as a color shift, perforation shift, inking irregularity or paper fold. A freak is a less severe and less valuable mistake than an error.

Oddity - a broad category of unusual variations. Examples: stamps used before their first day of issue, errors in postmarks, and semi-official separations such as the Kansas City roulettes.
(http://glossary.USStamps.org)