GP Posting to closed month

Karen
That's a real enigma:). Can you consult your system's user manual on exactly how it manages period closings. Some systems allow sub ledgers/modules to be closed independently of the GL, but once the GL closes, everything closes.
You may need to find out if the inventory module was re-opened for January.
If your user manual is unclear, use your vendor's support group to take you through the issue and find out how to prevent it in future. That support is normally covered in your annual maintenance fee paid to the vendor.
Regards
Len

You need to find out what that "something" is. If you follow Len's recommendations, you still need to track the transaction that posted and it will guide you on how to correct it. You may need to go through your transaction list. Usually they will show the posted date and transaction date. You can also compare balances of the closed month versus what it is today (month-end balance with the errant transaction).

I concur with the above comments, definitely needs to be tracked to understand what caused this to happen. Is it possible your system doesn't actually lock out any previous month posting? Being a Quickbooks user it can be a pain point, but in your case it sounds like a possible glitch, which is never good in accounting.