The log_contains_header.js test fails sporadically if both the first and second runs of mongod rotate the audit.log file at the same second, thereby causing the second rotation to skip renaming the audit.log file as the target file name already exists. This causes the second mongod to append log lines to the un-rotated audit.log file, instead of a blank file. When the test script looks at the audit.log file, it reads an unexpected header line, so it fails. Adding at least one-second delay in between the two runs will ensure this collision doesn't happen.