You probably did not extend the class table, but write the data at 0x80.
Item data is stored immediately after the class data.
08803D64 - 08807110 //Unit
08807110 - 08809B10 //Class
08809B10 - 0880B82C //Item
Breaking the end of the class table erodes the area of the item table.
For example, you are kicking the wall of the apartment and breaking into the next room.
The people in the next room will be astonished.