The following is -only- important when you’re single targeting trash that’s going to die very quickly. In a boss fight the loss of one tick is frankly insignificant. If you are part of the “trash is not important” camp.. feel free to skip this entry.
Anyway. Right now as an Affliction Warlock, if I have a single targeted mob I’ll use three spells (in no particular order) Haunt, Corruption and Shadow Bolt.
Corruption is an instant cast spell which tics every 3 second ticks (it does damage every 3 seconds). Assuming you’re in range, as soon as you cast Corruption it shows up as a dot on the mob. Three seconds after it lands on the mob, it will do it’s first damage to the mob.
Haunt has a 1.5 second cast time. Additionally, Haunt has a “flight time”, that is, the time between the end of the cast time, when the spell leaves your hands, and travels across the room. If you are further away from the mob then the flight time is longer. If you’re spec’d into Everlasting Affliction, then when your Haunt hits the mob it will refresh the duration of your Corruption.
The crux of the gotcha:
When Haunt refreshes corruption it essentially clips the dot. That is, if your corruption has 1.5 seconds until the next dot goes off.. and Haunt hits the mob.. then Haunt refreshes corruptions duration.. and the dot is then reset to not go off for 3 seconds.
So, if start off a fight with the mob by casting Corruption, then Haunt, then Shadow bolt.. your Corruption will hit the mob and will be there for the 1.5 second casttime of Haunt.. and then the flight time (probably ~.5 secs) and then Haunt will hit the mob and will refresh the Corruption duration BEFORE THE FIRST DOT ticks. So in essence, it will take 5 seconds for your first dot of corruption to hit the mob.
If you start off a fight by casting Haunt, then Corruption, then Shadow Bolt… Your instant cast corruption hits the mob -while- your haunt is in “flytime”. So Corruption hits the mob.. and is almost immediately refreshed. I say almost immediately.. I still think you’re looking at having corruption on the mob for .5-1 seconds.. so with the rest it would actually be ~3.5-4 second before the first tick of corruption will hit the mob. Not as “bad” as the previous example.. but still.. you get an odd reset of the corruption that just feels wasteful.
If you start off a fight by casting Haunt, Shadow Bolt, Corruption… then your haunt will be fully “on” the mob by the time your corruption lands. So there’s no “resetting” of the corruption dot. This seems like the best option to me.
I think grand total you’re looking at a difference of ~1k-2k -damage- on the mob in the first few seconds. So this is ONLY useful on mobs that you’re single target killing.