Yup, not good for them at all. Someone I sold a gecko too a while ago use to put pieces of fruit in with the worms in the leos cage (thinking this would keep the worms alive longer and gutload continually). Well the gecko ate a piece of apple and died 2 days later...moral of the story...only feed appropriate insects to them.
Apple seeds contain arsenic. I suspect that is what happened.
Awww that's sad... so how do we avoid that from happening if we suggest to people to put a piece of food in their mealie dish to make them more active to attract the leo? I know, obvious answer right make it bigger than the leos mouth right...