My best guess is you have a Latest Transaction date in the future and are running the Inventory report as of the Latest Transaction Report. You can check the date under Setup, Settings, Company, Information and the options on the Inventory report to confirm.

If you have not posted inventory transactions in the future, the results are accurate as of today as well.

I can see that the latest transaction is indeed in the future. How can I change this? Was this caused by user error?

I can't answer that. Since January 2015, I have had entries in my file up to November 2015 on purpose.

Run your Journal Entries report from the Reports menu for any dates Oct 27/15 to the Latest Transaction date and you will see the entries and if you check off the user, you will know who did it. Then you can determine if it was an error or not. If no entries show up, the entry(s) was in error and then corrected however the latest transaction date stays the same because the journal entries exist. Make sure you show corrections as well as the user.

Except for transfers / inventory adjustments, if you can't 'drill down' into the transaction, it has been reversed / adjusted.