Somekind of bug, it might occur somewhere else of everywhere else. I can't tell exactly, but this is where I've encountered it. Check the example below to reproduce it.
Create a new CardType, create an instance of that CardType.
Edit the structure of that instance and add {{+some name with spaces | type: Basic}}
Now go to that card... notice the name of the plus card says "Add+some name with spaces" in normal form. Click Add and a small window appears. Now instead of Submit click Cancel.
You are back to the original card. Notice the like that spelled "Add+some name with spaces" now spells "Add some_name_with_spaces"
Clicking Add again will create a card with "_" instead of spaces.
Good news: this is fixed in the latest code, too!
Wow great!
this code is released now.