what you're describing on a journal entry doesn't sound very likely for pure foreign currency exchange. I can see it happening on an advanced intercompany journal (because there is a per-line exchange rate), but you would definitely expect to see some audit trail on the line if that was the case.
Are you looking at the journal at translated rates? i.e. you are looking at its effect in a different currency to the base currency of the subsidiary? If that's the case, then yes you can have a maximum of three different rates on the transaction (one for current account types, one for average account types, one for historical account types).
Depending how many times you run the period close process and the intervening transactions, those rates could change dynamically throughout the month
The above assumes you're not using multi-book