This is a must read for all referees. This article explains the extremely rare situation of when a defensive player is off the field of play but keeps an attacker onside. If this happens to you, and you know how to react, you will look like a hero (at least to one team!)
This provides some clarity to the situation. From the 2009 USSF Memorandum:
USSF Advice to Referees: The new text more explicitly describes how referees are to consider the location of a defender off the field when deciding if an attacker is or is not in an offside position. A defender who is off the field with the referee’s permission (and thus cannot freely return to the field) is not included in determining where the last and second to last defenders are located. A defender whose position off the field was not with the permission of or at the direction of the referee is deemed to be on the goal line or touch line closest to where the defender left the field and must therefore be considered as though still on the field. Furthermore, if the departure from the field is “deliberate,” (i.e., other than during the normal course of play), the defender is to be cautioned for the misconduct.
Ignorance of the law excuses no man. -- John Selden, English historian