We get this on occasion, mainly with AP voucher entry.
My research at Oracle on this shows in previous versions of E1 it was to do with the update of the AP transaction being done asynchronously and the GL transaction being done synchronously, and the user hitting OK on the GL transaction before the AP transaction was completed.
This does not seem to strictly apply to E9.1, as the whole way it handles asynchronous processing on the OK button has changed, but it may be worth checking with the users if they are pressing OK on the second (GL) screen quite quickly, or if the system was performing particularly badly at the time.
Another possibility would be a timeout or connection issue. I've adjusted some timeout values on the servers, and waiting to see if I continue to get these calls.
Hard to test, as we can't reproduce it, it just seems to happen randomly.