Fast forward to this year— a WordPress update caused the theme to start generating errors. The errors were not the result of our hacks, but were in the original theme. The theme author gladly supplied me with the latest error-free version of his theme, but because I did not document all of our hacks, we would essentially have to start over.
Lesson 1: Never hack a WordPress theme.
Lesson 2: If you ignore lesson 1, copiously document every change, because you may need to re-apply all hacks in the future.