diff --git a/justfile b/justfile
index ff75b8d..a81c78b 100644
--- a/justfile
+++ b/justfile
@@ -398,6 +398,11 @@ powerloss-image:
 
 # Real Linux binary + RocksDB + ext4 + kernel NBD. Expects `just simulator`;
 # Go/Zig/uv and Docker are all forced to their existing offline state.
+#
+# A killed run can leave its NBD device claimed inside the Docker VM: the next
+# run then dies with "nbd: nbdN already in use" even though /sys/block/nbdN
+# reports size=0 and no pid. Point STREAM_POWERLOSS_DEVICE at another device
+# (nbd0..nbd3) rather than reading that as a Stream failure.
 powerloss-oracle:
     zig build write-sample -- /tmp/stream-powerloss-fixture.jss
     rm -rf /tmp/stream-powerloss-linux