When trace output is piped to command and this command terminates prematurely,
strace treats pid of finished pipe process as one of children it have to handle.
As result, strace either fails with "unknown pid" message or (in -f mode) loops waiting for non-existent child.
$ strace -o '|true' true; echo rc=$?
unknown pid: 12345
$ strace -f -o '|true' true
[strace falls asleep forever]
Created attachment 135392 [details]
these bugs are fixed upstream in the coming 4.5.15 release
4.5.15 in rawhide and in updates for fc5 and fc6 fixes this.