Hey,
Knight/Druid has the ability to shield groupmembers and block dmg for them. If you shield someone, one of your Elite skills, Holy Flank(1490517), will buff all shielded party members for 15 secs, which gives everyone a patk/matk buff(1501171). If the buff runs out, and you are casting something (like music) it will cancel the cast, which can be fatal if the casts have to be in time. It might not be a big deal, but i ran a few times and tested it and i got my music canceld often enough, and even some healers got their revival canceld which can result in a group wipe.