diff --git a/specs/altair/beacon-chain.md b/specs/altair/beacon-chain.md
index 59b11c8e4..0855d6f11 100644
--- a/specs/altair/beacon-chain.md
+++ b/specs/altair/beacon-chain.md
@@ -277,6 +277,9 @@ def get_sync_committee_indices(state: BeaconState, epoch: Epoch) -> Sequence[Val
     """
     Return the sequence of sync committee indices (which may include duplicate indices)
     for a given ``state`` and ``epoch``.
+
+    Note: This function is not stable during a sync committee period as
+    a validator's effective balance may change enough to affect the sampling.
     """
     MAX_RANDOM_BYTE = 2**8 - 1
     base_epoch = Epoch((max(epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD, 1) - 1) * EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
@@ -310,6 +313,9 @@ def get_sync_committee(state: BeaconState, epoch: Epoch) -> SyncCommittee:
     ``SyncCommittee`` can also contain duplicate pubkeys, when ``get_sync_committee_indices``
     returns duplicate indices. Implementations must take care when handling
     optimizations relating to aggregation and verification in the presence of duplicates.
+
+    Note: This function should only be called at sync committee period boundaries, as
+    ``get_sync_committee_indices`` is not stable within a given period.
     """
     indices = get_sync_committee_indices(state, epoch)
     pubkeys = [state.validators[index].pubkey for index in indices]