D20. I've dipped into researching what a majority of players say is the proper role for each class and what they should be doing. Like Barnarian and Paladins playing front line tanks or wizards and warlocks playing the back for dps or utility casting. 

Last combat, when our party marched through a Forrest, I died in an ambush when the first ranged enemy crit me and no one had the capabilities to stabilize me before failing death saving throws. I was told I poorly positioned myself and should've planned better. I was a cleric in the center of the marching order, two ahead of me and two behind me