@Chip
The surefire way to reproduce this is to
1. Install
Advanced Filters version 1.3.3.0
2. Get a container in your inventory (writ box, PTS template item distribution, etc.)
3. Hover over the container and do not move the mouse at all
4. press 'E' or double-click to open the container inventory
5. Press alt to close the container inventory
6. Press 'E',
not double-click, to open the container inventory again
7. Error displays
Between steps 5 and 6 is when an addon call to PLAYER_INVENTORY:UpdateList is triggered with the cursor over an item slot (thereby causing that control to become untrusted somehow)
My fix cycles the mouse's state after the inventory update. This apparently allows the moused over control's integrity to be re-verified.