I have been tanking my entire eq1 and eq2 career and consider myself pretty damn good at it. My healers are rarely, if ever, attacked, but anyone who has played a tank knows inevitably there will be one of those rare situations when he comes under fire. Unfortunately 1 out of 10 of those healers decides it is a brilliant idea to run in a circle. STOP IT!!!!! You are not doing anybody any good and are making a bad situation worse because running in circles makes it difficult for the tank to target the mob that is on you. As wierd as it may sound a far better solution is to sit there and yell to the tank for help. He can then quickly come over and take the mob off of you. You could even score some tank brownie points and take the mob right to him. These people represent a small minority of casters, but they represent a huge amount of group wipes.
P.S. To all you finger wigglers and Stabbie Mcgee's show some patience. In eq2 your tanks are dealing with 4 mobs and up at a time. Give them a second to get good aggro so clerics don't have to waste mana on healing stupidity.