Wow, good catch! That was totally unintentional, but it seems to have been that way since before v1.0. I'll fix it in the next update.
Viewing post in Hints and Bug Reports [avoid major spoilers]
Extremely minor nitpick with the new version (I'm planning to run it with other people, this won't stop me though ;p): there's two Ds in puzzle 12 that are interchangeable. I don't remember this happening anywhere else in the puzzles, and it's not like any rule is against that, but if that matters to you, it's there.