That's a similar case, with the difference that there is a race condition here. "some chain" and "tee" are started in parallel, and it's a random process who co

That's a similar case, with the difference that there is a race condition here. "some chain" and "tee" are started in parallel, and it's a random process who comes first: "some chain" for opening inputfile for reading, or "tee" for opening (a new file, but with the same name) it for writing. You can provoke it by echo a > a (sleep 1; cat a) | tee a very likely "a" is empty now.