I've re-read One Piece multiple times and there's only one thing that could fall under my definition of Plothole.
It was about Madam Shyarly's Scrying.
1) When she's asked when Luffy will destroy FI she says "I don't know, I can only see WHAT happens. I don't know if it'll be tomorrow or a year from now".
2) When Roger asks baby Shyarly about Poseidon's birth she says "In 10 years".
So that leaves me with:
A) The translation on the Fishman Island prophecy is wrong. Her ability can see what and when an event occurs. So she's not establishing a universal rule on all of her prophecies. She's just referring to that specific prophecy, that she only knows what happens regardless of time frame.
B) When she was young her ability could discern both content and time of prophesied event. And as she got older the ability weakens losing that added perk. (But this still needs to be mentioned in the story if it's the case so for now it's a possibility)
C) Shyarly is lying about not knowing the time of Luffy's destruction of FI (Very very very very unlikely)
D) It's a Plot Hole.
For now I won't legitimately say it's a plot hole until I can find the raw of said scenes and double check with a qualified translator whether the wording in the Fishman's Island scene was specific to the timing of Luffy's prophecy, or a general rule related to her Scrying ability.