Just a little example:
Jindo Krey in HM FE tends to have random-aggro and jumps back and forth between DPS.
Last time I tanked this FP we had a random Sorc-DPS with us.
He decided to stay back at the ramp where you approach him while my two guildies were at the cannon.
I told this guy multiple times to get closer to us since I could get at most ONE attack off before JK jumped away again.
It's stuff like this that makes me hate tanking with PUGs.