The Second Sting Hurts the Most

Alan and Nathan are playing in a Kaladesh/AEther Revolt Limited PPTQ. Alan casts a Foundry Hornet, then attacks with an Eager Construct that has a +1/+1 counter on it. Nathan controls his own Eager Construct, but chooses not to block with it. During his second main phase, Alan casts a second Foundry Hornet.

Alan looks at this hand for a few seconds, then says “Umm… I guess it’s… WAIT!” He then reads Foundry Hornet again. “Your Eager Construct dies.” Nathan calls you to the table and explains that the first trigger is missed, since it wasn’t mentioned during first main phase or combat. When you ask Alan, he tells you he forgot about Foundry Hornet’s trigger and didn’t realize until he played a second one.

Yes, we know that Kaladesh/AEther Revolt isn’t the current PPTQ format. You can ignore that when giving your answers.
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While Alan does appear to have forgotten one of his triggers, he has not missed the opportunity to acknowledge it, as the first time it is relevant to the game state is now. Treat the first Foundry Hornet trigger as resolved, and the second as still on the stack, Nathan can respond if he would like. Eager Construct will die when it resolves if nothing else happens. There is no infraction, as no triggers were missed.