libpredict registered the tbc spellcast and combat log events on every
client and relied on vanilla never firing them. A vanilla client with an
API backport such as ClassicAPI fires UNIT_SPELLCAST_START as well, without
UnitCastingInfo, so every cast raised "attempt to call global
'UnitCastingInfo'". Register those events on the tbc client only; vanilla
keeps using its own SPELLCAST events.
shaga
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