Since it is document-specific it doesn't really sound like a setting you 've changed. That sort of thing would be handled in a keyboard shortcut set, and as far as I know they affect all documents as they are opened.
So there may be some sort of document correuption. Make a backup copy of the file for safety, then open the file and export to IDML. Open that and see if the problem is fixed.
Is this a document that you created yourself or one you inherited from another source? In either case check in the Text and Tables section in Keyboard Shortcuts (accessed through the Edit Menu). Click on the entries for decreasing point size and see if the Current Shortcut for the document is showing as a "T". If that is the case change it back to the appropriate keyboard shortcut.