IIRC Golshi pulled this exact same prank on Flash in one of the in-game rival event stories. Though it went somewhat differently. Maybe it was just mentioned in passing on her home screen convo or something?
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Anyhow, Flash's hobby also happens to be baking. And yes, she's very fussy about the measurements, down to the specific exact gram.
Updated
Phew...She'll deffo notice it! It's Flash we're talking about here!See, she's the sort of person who would micromanage things down to the gram!
500K Yen
Savings
Piggy Bank
whisper
Every time Flash finishes a race...What an asinine thing to do...
heft
...she'll put a 500 yen coin inside her piggy bank.
whisper
Putting a five yen coin into Flash's piggy bank, such... inanity...