diff --git a/bin/compose b/bin/compose
index 6577d6d..18427c9 100755
--- a/bin/compose
+++ b/bin/compose
@@ -386,8 +386,18 @@ clean_unused_sessions() {
         [ -e "$f" ] || continue
         is_volume_used "$f" && continue
         ## XXXvlab: the second rmdir should not be useful
-        rm -f "$f" >/dev/null || rmdir "$f" >/dev/null || {
-            debug "Unexpected session remnants $f"
+        [ -d "$f" ] && {
+            err "Unexpected directory as session remnant $(printf "%q" "$f")" >&2
+            echo "  - can you contact support to report this issue ?" >&2
+            echo "  - as a workaround, you can remove it manually using:" >&2
+            echo "" >&2
+            echo "      rm -rf $(printf "%q" "$f")" >&2
+            echo "" >&2
+            return 1
+        }
+        rm -f "$f" >/dev/null || {
+                err "Couldn't delete $(printf "%q" "$f")" >&2
+                return 1
         }
     done
 }
@@ -963,11 +973,16 @@ mk_docker_run_options() {
         )
     fi
 
-    clean_unused_sessions
+    clean_unused_sessions || return 1
     filename=$(mktemp -p /tmp/ -t launch_opts-XXXXXXXXXXXXXXXX)
     p0 "${docker_run_opts[@]}" > "$filename"
     hash=$(hash_get < "$filename") || return 1
     src="$SESSION_DIR/$UID-$hash"
+        if [ -d "$src" ]; then
+            err "Unexpected directory found in '$src'."
+            return 1
+        fi
+
     dest="/var/lib/compose/sessions/$UID-$hash"
     additional_docker_run_opts=(
         "-v" "$SESSION_DIR/$UID-$hash:$dest:ro"
@@ -984,8 +999,17 @@ mk_docker_run_options() {
         p0 "!env:$var=${!var}"
     done >> "$filename"
 
+    if [ -e "$src" ]; then
+        ## compare content of $src and $filename
+        if ! diff -q "$src" "$filename" >/dev/null; then
+            return 0
+        fi
+        warn "Session already exists but content is different. Squashing."
+    fi
+
     mkdir -p "$SESSION_DIR" || return 1
-    mv -f "$filename" "$src" || return 1
+
+    cat "$filename" > "$src" || return 1
 
 }
 
@@ -995,6 +1019,10 @@ load_env() {
     if [ -z "$COMPOSE_LAUNCHER_OPTS" ]; then
         mk_docker_run_options "$@" || return 1
     else
+        [ -d "$COMPOSE_LAUNCHER_OPTS" ] && {
+            err "Variable \$COMPOSE_LAUNCHER_OPTS provided but it points on a directory."
+            return 1
+        }
         set_os || return 1
         while read-0 opt; do
             if [[ "$opt" == "!env:"* ]]; then