There is another warlock in my guild, who shall go unnamed to protect the oblivious, who has decent gear and shows up for raids fairly regularly, but who has always struck me as doing completely inadequate dps. He is Affliction (and the reason that I am not, since THANKS BLIZZARD Shadow Embrace is still busted), but I regularly trounce him on damage meters, on fights with tons of movement like Grobbulus and on fights where we stand still and beat the living crap out of things like Patchwerk. I'm not particularly better geared than this guy, and Affliction is widely hailed, both by my peers and on numbercruncher sites like Elitist Jerks, as a vastly superior raid spec.
This has baffled me for weeks now, and I figured out why today.
Being that I am Destruction, the best way for me to squeeze out that extra little bit of dps involves not only watching whether my own Immolate is on a target, but whether someone else's Immolate is up there as well, so I can throw another Incinerate when it's debatable whether recasting Immolate will overwrite the last tick without having to worry about whether Incinerate will grab the bonus damage. So I paid particular attention to the Immolate buffs on the boss, and lo and behold... Not only did the other Immolate never fall off, but it was being refreshed before it was done ticking.
This guy is Affliction, remember, and he had been refreshing not just Immolate but all of his DoTs before their final tick had gone off.
I feel it is my place to say, if you are reading this post, DO NOT DO THIS. EVER. You hear lots and lots of chatter about how it's very important to keep your DoTs up all the time, and it is! However, what people mean when they say this is that it's very important that you refresh your spells as soon as they fall off. Never refresh a DoT before it is done ticking. That last tick of damage you're overwriting is damage you have already paid for, both in terms of mana cost and in terms of time spent casting. Every single time you overwrite a DoT that still has a tick or two left, you are wasting that damage. You are robbing yourself of that DPS.
I informed the warlock in question of his error in private following our raid today, and I expect to see an astronomical increase in his DPS tomorrow. Maybe I shouldn't have told him so he could continue making me look good by comparison.