Why is it that all my KDE-related postings seem to be about disabling annoying user interface misfeatures?
Today's has to do with the Ctrl-Shift-Underscore (Ctrl-Shift-_) key combination. This is used for "undo" in EMACS-derived text editors, including JOE. In recent KDE versions, Konsole has started trapping this key combination for "reduce font size." So you're merrily editing away, you try to use the undo command, and suddenly your font has become smaller. To make matters worse, it is a known bug that the key combination Ctrl-Shift-+, which is supposed to be "enlarge font size," doesn't work. So not only can you not undo editing changes anymore, but you can't undo the font size change either. Solution below.
It turns out there are at least three places KDE-related keyboard shortcuts can be configured. The one that's relevant here is on the Konsole menu bar, under "Settings"->"Configure Shortcuts..."; that gives a scrolling list, of which the relevant item is "Shrink Font." This dialog box uses a weird nonstandard widget; click on an item, and it expands to a multi-line pop-up thing that still seems to be part of the list; to disable a shortcut, choose the "Custom" radio button and make sure the button next to it reads "none." If it's already set to a non-"none" custom setting, you can clear it with the delete icon. There is no clear way to close the pop-up thing, but it seems to save its value when you leave the dialog or choose a different item.
I took the opportunity to delete all the other Konsole-specific hot keys; I don't think I've ever used any of them on purpose, and things like Ctrl-Arrows to switch between tabs have occasionally given me unpleasant surprises when I hit them accidentally. Disabling just Ctrl-Shift-Underscore, though, will make my use of Konsole a lot pleasanter.
For reference, the other two places I'm aware of where keyboard shortcuts are configured: first, in Konsole, under "Configured Current Profile"->"Input" (these are different because they adjust the terminal emulation: key combinations configured here expand to escape sequences sent to the underlying application); and second, in "System Settings"->"Common Appearance and Behaviour"->"Shortcuts and Gestures." The "Shortcuts and Gestures" seem to be global to KDE applications, but many KDE applications (such as Konsole) do not actually use them.