If it restarted every few hours and the test executed once a minute, then the bug would appear only once in a hundred executions. It's a good post, I think I could have made the same mistake easily.

It takes me about 10 runs to get a line of output without the grep. Sometimes less. Try it :)

EDIT:

test.sh:

  for i in {1..1000}
  do
  ps | grep grep | wc -l
  done

./test.sh | sort | uniq -c

  13    0
  987   1

Which would indeed suggest 1.3%. Likely dependent on how quickly you're running it, if it's in memory already etc etc

I'd never noticed it. I worked in a unix shell for a while, but I used to use ps piped to grep. But I suspect I often used 'grep -v' to filter the output anyway when I really needed just the non-grep lines.