Rogue currency exchange rate

[Fred Bone]

If I go into Price Editor and expand the entry under "Currency" for CHF, 
I see a line showing "source: user:price-editor", "type: unknown" and 
"price: 0.0000000".

I could just delete this line, but I have a suspicion that somewhere 
there's a transaction using it.

1. What would be the effect of deleting an exchange rate that's linked to 
some transaction?
2. How can I either locate the affected transaction, or establish that 
there isn't one?