This game, like all the others, functions trough thousands and thousands of programming lines and commands, all very sustained and comprehensive constructed into the WoW system. When something goes wrong, you check the code and see if there's anything that isn't working as it should. However, around 70-80% of the times, this doesn't occur because of clueless calculations or mindless actions - it's done on purpose.
I will state some examples that I remember (no particular order):
- Back in 4.x.x (Cata), Kaelthas in TK was an average solo kill for most classes with good gear and skill; in some middle patch, his HP skyrocketed by a % making the fight at that time way more harder than what was supposed to be.
- Sha of Anger and Galleon mounts non-existent. The outburst in the forums stating that absolutely no one had yet managed to get a drop
of these 2 world bosses mounts - guess what, after a non-existent statement, they suddenly started to appear (this was after MoP launched, which leads me to believe that they wanted to preserve the rarity for 1 patch and make people go there just to kill them even if they wanted the gear or not).
- PTR after Dragon Soul and before MoP - massive gold nerf across the board to all the raid bosses when killed solely; only after players went to PTR, entered the raids, killed the bosses, traveled to other raids and still got the same gold, they started to realized that something was off and started complaining. Only then, a Blue poster explained that was a change coming to stay (again with no statement, only after we, players, discovered it by ourselves).
- Another boss, Alysrazor in FL. Before 5.4, it was a walk in the park, now go try it yourself doing the "exit arena tactic" and you will see that Harsh Winds actually only stops at 12 stacks instead of 6-8 (I think), stacks more fast and damages you for > 100K after the 7-8th stack making it really harsh for some classes.
- Other notable example, "increased drop chances". After a patch is released, you read it and think to yourself: why wasn't a bigger drop chance in the first place? Why wait for a single patch to implement it if it only requires a weekly maintenance? Again, to prevent massive income of such objects and increase your playtime. Want an example? Zandalari librarian cards required for the achievement and the meta attached. Maybe in the future they will also increase the spawn chance and/or rare spawn timers/drops and/or archeology related stuff.
- A general thing that happens in PTR: big amount of nerfs / buffs to all classes, numbers tweak, etc. The majority is set to be in stone, the rest is too see, learn and react the community reactions to it. Most recent example, PW: Barrier increased to 100% damage reduction - do you really think the amount of skill and technique that the programmers there have, would let them slip something like this, most notably such a round and numerical value like 100? Why not 64.81?
But of course there are normal slips here and there like the mount from Raggy which had 100% drop chance on normal for a few weeks, the bug that allowed infinite Tol'Vir digistes just by changing zones (blame CRZ), Muradin's Favor incorrect skin and a plethora of other (in)famous stuff.
It might sound awkward at first, implausible or maybe smell like conspiracy theories, but this is what I think from personal experience, observance and playing the game.
I'll add this for giggles:
http://www.youtube.com/watch?v=Ssnw2GA657s