I Am Stuck Running
In most cases, pressing Ctrl-R to toggle run mode will get you back into walk (though it may in some cases be necessary to hit Ctrl-R more than once).
If you are wearing a scripted AO (animation overrider), turn off any “running” features it has.
If you are using the Firestorm built-in AO (animation overrider), make sure there are no running animations in the “Walking” or “Standing” categories.
In one specific case, the above will not work. Specifically:
You have Preferences → Move & View → Movement → Tap-tap-hold to run enabled.
Press shift, then tap-tap-hold the left or right arrow key
That gets you running sideways. If you release shift before the arrow key, you will be stuck running.
To stop running, you have to repeat the sequence: press shift, then tap-tap-hold left or right arrow.