Additional steps are required to prevent tampering because of a type of attack unique to multi-layered encryption in anonymity networks called tagging attacks, where tampering is done to cause the circuit to disconnect at a specific relay.

@EllaRose From the classic collective of 23 cypherpunks raccoons (reminds me a lot of SqueamishOssifrage, actually): lists.torproject.org/pipermail/tor-dev/2012-March/003347.html