We have Allow Transaction Date Outside of Posting Period set to Disallow and have had it set that way forever. I checked the Accounting Preferences audit trail and that setting has not changed, nor has any other Accounting Preference changed for over 2 years. However, the system somehow let a user hand-key and save a transaction with a date in a prior period, similar to allowing a posting date of 4/30 for May 2024. The Posting Period dates have not changed on the Period records themselves are set to calendar month dates. No relevant scripts and only a simple approval workflow running on this transaction type (bills). None of those should be able to override that setting anyway. Anyone know how this might have happened? One transaction isn't a huge deal, but it makes me nervous there are others I haven't found.

For starters I would check what combination of user/role/context was involved.

UI and a basic non-Admin user. That setting should stop even Admins from posting a transaction in that way though.