I need to filter a live log stream for only relevant events and quickly hit an issue that I wasn't expecting. The grep in my pipe chain was waiting until it received all the output from the prior command before it began to attempt to filter it.

Reading through the grep man page I came across the --line-buffered flag which provides exactly what I needed. I wasn't using the tail command but it serves really well in this situation to demonstrate the use:

tail -f /var/log/maillog | grep --line-buffered -i error

Hope this saves someone a headache in the future!