GNU /usr/bin/time -o puts a status line first when rc != 0

GNU time with -f FORMAT -o FILE writes an extra first line, 'Command exited with non-zero status N', before the formatted line whenever the child exits nonzero. So a benchmark harness that does read wall usr sys rss rc < time.txt reads that words-only line and emits garbage fields (e.g. invalid JSON) for exactly the runs you most need to see: negative tests and failures. Read the last line instead (read ... < <(tail -1 time.txt)), or parse by format marker. A stub run where the fake command exits nonzero catches this before a real slot.