Hash-based signature keys must never sign two different things for the same slot.
The idea: A hash-based one-time key signs by revealing part of its secret, so signing two different messages with the same one-time key can reveal enough for someone to forge. XMSS gives each slot its own one-time key, which turns the rule into "never sign twice for the same slot". Validators already avoid exactly that, because double signing is slashed.
Why it matters: Validators already have to avoid that today, so it fits well.
On the map: XMSS-style keys are stateful: signing twice with the same one-time key enables forgeries
Hash-based signatures like XMSS use one-time keys[1], so they are stateful: a signer must never sign twice with the same one-time key, or an attacker can forge signatures.
Ethereum validators already must never sign conflicting messages: two blocks for the same slot, or two votes for the same epoch, get slashed today. So slot-indexed hash-based signatures fit naturally[2]. Care is still needed around backups, failover machines and restores.
If any of these breaks, this breaks too.
If this breaks, these break with it.