Kaervek, the Punisher exiling a card that is still on the stack
Opponent had Kaervek, the Punisher on the field. He has a triggered ability, which triggers on committing a crime. The triggered ability exiles a black card from your graveyard and lets you cast it.
The opponent had no black cards in the graveyard, but cast one that triggered Kaervek's ability. Because this is triggered by committing the crime, the spell that triggered it is still on the stack, and according to 608.2m the spell isn't placed into the graveyard until it finishes resolving. Kaervek's trigger is placed above it on the stack, and therefore resolves first. However, the system allowed him to exile and copy the spell that was still on the stack as if it were already in his graveyard.
The Kool#25465
shared this idea
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Rezzahan#77802
commented
Did your opponent cast a spell? Or did he activate an ability of a card in his hand that put it into his graveyard? For example, activating the ability of Harvester of Misery will put it into the graveyard before the crime trigger from Kaervek goes on the stack. Thus it can be cast via that trigger, in addition to its ability.