I've been doing some work with a US-based company as they implement Microsoft Dynamics GP in Europe for one of their subsidiaries (we do a fair bit of this type of work for expertise with VAT and Intrastat). One of the issues they asked for assistance with was a problem they had while importing transactions.
All of the transactions which were in the functional currency, imported without a problem, but all of those in other currencies failed.
The cause of the problem was actually quite a simple one, and one I have seen before with other clients. On the Multicurrency Exchange Rate Maintenance window, there is an Expiration date field: