3,966. An Interposition

Hilchos Klei HaMikdash 10:5

Just as a kohein who serves lacking garments is subject to a Divine death penalty and performs an invalid service, the same is true of one who wears extra garments, such as two tunics or sashes. If a regular kohein serves wearing the garments of the Kohein Gadol, he desecrates the service and is subject to a Divine death penalty.

Hilchos Klei HaMikdash 10:6

Leviticus 16:4 says that the kohanic garments shall be “on his flesh…and he shall wear them.” We see from this that nothing may be between the kohein’s skin and his garments. Even a single thread, dirt or a dead insect between the garments and his skin would be considered an interposition, thereby rendering his service invalid. For this reason, a kohein may not serve while wearing his arm tefillin, which would be an interposition. The head tefillin do not interpose, so they may be worn while serving if he so desires.