Warlock Multiclass Options: A Complete DnD 5e Guide
In D&D 5e (2014): Warlock Pact Magic spell slots are separate from the standard multiclass spell slot pool. You cannot use your Warlock slots to cast spells from other classes, and vice versa.
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A factor which could serve to automatically balance this multiclass is the fact that Warlock spell slots are generally not actually able to be purposefully consumed until all regular spell slots have been expended.
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A half-caster or third-caster could, thanks to switching to the multiclass table but not actually getting any spell slots out of their dip, theoretically lose some spell slots otherwise.