s/do things/accomplish tasks/ above.

There are a limitless number of input combinations, and there's potential for those to interact in unforeseen ways. However, the ideas encompassed in drafting using a CAD program were considered by the developer and the UI for doing that was considered by the developer and designer. Perhaps a "better" UI would have indicated which features were compatible with the active modifiers in the UI.

There certainly are bugs that will not be caught no matter how much manual or automated testing is done. Front end code has more entropy in its inputs and perhaps will have to deal with more unforeseen circumstances.