Prolog programmers often face "coding horror" when using impure language constructs that create defective programs. Prolog's declarative nature gets compromised when developers use cut operators, non-monotonic constructs, or global database modifications. These practices lead to programs that either report wrong answers or fail to find all intended solutions, undermining the core value of logic programming.

The horror manifests in several ways: lost solutions from using cut operators, non-monotonic constructs, or global database modifications, hidden dependencies from assertz/1 and retract/1, and poor debugging when printing directly to terminal rather than letting the toplevel handle output. The factorial example demonstrates how these issues create programs that fail on general queries, limiting Prolog's expressive power and making code harder to maintain.

Writers recommend embracing pure, monotonic Prolog constructs instead of outdated features. Using constraints like dif/2 and meta-predicates like if_/3, along with declarative arithmetic (#= instead of is), produces more general programs. The improved factorial example shows how these changes enable correct handling of the most general query while maintaining acceptable performance through cleaner code structure and better adherence to Prolog's relational foundations.