Why doesn't my 'E' key work to begin a workout in Zwift?

You may find that sometimes you try to start a workout by using the standard 'E' key on your keyboard, but it doesn't seem to work.

What?! Why? It's always worked before!

But wait, don't leave just yet! #RageStay

Barring all of the first-level troubleshooting steps:

1. Did your keyboard's cable become detached from the PC's USB port?
2. Did your batteries run low on your wireless keyboard?
3. Did you perspire on the keyboard so much it has just stopped working?
4. Are you running something like AutoHotKey that steals the 'e' key away for another purpose or shortcut?

If you've verified all of those things, there's just one more:

Once you've joined an official Zwift event in-game, the 'E' workout menu shortcut key is disabled, so you will be unable to join a workout while actively participating in that event. You'll need to quit the event first using the "Quit Event" button in the lower-left corner, before beginning your workout.

If you think about it, you wouldn't want to be actively in a workout while you're in a ride event, because your efforts at each segment of your event would not be achieved, as you struggle to maintain position in the pack of your ride or race.