Generally, when you suspend a G/L, journal activity cannot occur on a suspended G/L account.  Any balance in the account will remain, but the account cannot be used for any posting.  However, there may be some places, such as configurations driven by the specific suspended G/L (that were configured prior to the G/L being suspended) that still will use the suspended G/L.  

In this case the transaction was posted via a Misc. Posting Code.  These are used when posting things such as misc. receipts via teller and Phone Operator, as well as misc. advances via teller, misc. expense checks, etc.  If the MSR uses code 999 and tries to enter the GL manually, they will be warned about the suspended status of that GL account.  However, if you use a code that was tied to that GL, the code will still allow posting.  

Therefore, whenever you suspend a GL account, you must also verify any misc. posting codes or other configurations that have been tied to that GL, and suspend or adjust those posting codes as well. Otherwise they can be used to post coded account adjustments, misc. receipts, or other transactions.