Like in the subject I got different result from the same grep command.
The following command is run on a central server which query the other servers via ssh.
ssh root@$domain "grep `date +%Y/%m/%d` /var/log/slave.log* | grep 'Blacklist\ requests,' | tail -1" > "$current_dir"/servers_result
On different servers with the same Debian 6 in some case I've got no matches, instead in other case I've got the matches. I've checked the log and I can see that the entry exists.