Ever since patch 2.3, a number of players have been having a lot of trouble with their instant abilities, especially in PvP. They're finding they get constant "spell is not ready yet" and "target is out of range" errors whenever they mash their buttons. The main source of this problem has to do with a change to the way your computer communicates to the server what your character is doing. It's not very easy to explain, but I'll do my best here. First, I'll explain how the change affects longer spell casts, and then afterward look at how it affects instants.

Here's how things worked in patch 2.2:

1. I press a button to cast a spell or activate an ability.
2. My computer says, "Hey! Realm server! David wants to cast Frostbolt!"
3. The realm server says "Okay!" 300 milliseconds or so later (this round-trip communication time is your "latency").
4. My computer then starts a 1.5 second global cooldown, and shows me the Frostbolt casting bar.
5. I cannot use any other abilities from the time I press the button to the time my Frostbalt casting finishes, unless I manually cancel the spell (as with a /stopcasting macro), or unless the server tells my computer, "Okay the spell is finished already!" or "Whoops! That spell got interrupted!"
   
6. Either way, without a manual interruption on my part, I'm waiting on the server to tell me the outcome of the first spell before I can tell it to start casting the second.

Here's how things work in patch 2.3:

1. I press a button to cast a spell or activate an ability.
2. My computer says, "Hey! Realm server! David wants to cast Frostbolt!"
3. My computer goes ahead and starts the global cooldown for me, assuming the Frostbolt will succeed.
4. The realm says "Okay!" 300ms later, and the casting bar shows up.
5. Alternately, if there's a problem, then the realm says "No way, silly! David isn't finished casting Fireball yet! Wait a moment to try again, and cancel that global cooldown while you're at it!"
6. Either way, I can send my commands to the server whenever I want, as long as my global cooldown isn't currently active -- and if it gets activated too early, I just have to wait for the server to tell my computer to cancel it before casting another ability.

Sounds fine, right? Before, we had to wait for latency between our computers and the realms in order for any spell to go through, but now we just have to wait if we press a button too early.

Your character can be less effective if you mash buttons in 2.3, according to Altitis. You can also stop using the /stopcasting command in most macros.

In the 2.3 PTR patch notes, "client spell cast requests are now sent to the server even if your player is already casting another spell. This eliminates the need for /stopcasting in macros to compensate for latency."

What this means in English is that outside of global cooldown, your computer will try to send a new cast command to the server when you press a button, no matter what.