Scoreboard teams' friendlyfire option isn't respected when a player attacks a mob.

If you add yourself and a zombie to a team with friendly fire off, you can still attack the said zombie.

Summon a Zombie (/summon Zombie)

Create a new scoreboard team (/scoreboard teams add 1 1)

Make everyone join the said team (/scoreboard teams join 1 @e)

Turn friendly fire off (/scoreboard teams option default friendlyfire false)

You can still damage the zombie.