There's latency forgiveness built into autocasting that is similar to autoattacking that helps ensure you'll be casting at the maximum rate regardless of your ping. This can cause cooldowns on autocasting that are up to a half second faster than the standard global cooldown, and doing any action (or just having an update at the right time) can trigger the autocast to occur prior to what the client thinks the current global cooldown is, thus refreshing the global cooldown spiral fade before it looks like its time is up.
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Computing the probability that at least one of the following events will occur:
P(a or b ... or z) = 1 - P(!a and !b ... and !z)
Probability
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