Another thread reminded me of a discussion involving an odd quirk in the rules.
A lineman, by definition, must be breaking the plane of the snapper's waist and have his shoulders approximately parallel to the end line. A back, by definition, must not be breaking the plane of the waist of the nearest player on the line.
The situation. A85 lines up as a slot receiver (ostensibly covered by A88 who is legally on the end of the line as a lineman) far enough forward that he is breaking the plane of the waist of the nearest lineman (A88 and A85 are not separated by a "blade of grass"), but then turns his body backwards so that he is clearly facing the QB (in shotgun formation). His shoulders are beyond perpendicular to the end line. In this stance, A85 is meets neither the definition of a lineman nor of a back.
The argument came in where Group A stated that he is not a lineman, therefore must be a back. If there were four other backs, you would have a flag for an illegal formation (with only 6 linemen). Group B countered that the rules for both illegal formation and pass eligibility only state that relevant players be "on the line" not specifically "linemen", and that a back must not break the plane of the waist, which A85 clearly does, so since he is not a back, he must be a lineman.
Is it possible that A85 could be the cause of an illegal formation (for not being the 7th lineman) and if he ran downfield in a pass route, also be an illegal man downfield?