A common programmer error is to leave out one or more clauses in the antecedent of a rule. This causes the rule to be over-broad in its application and may cause it to subsume other rules.
Example: SACON structural analysis consultant[ Bennett, J. S. and Engelmore, R. S., ``SACON: a Knowledge-based Consultant for Structural Analysis'', Proc. IJCAI-79, pp. 47-49, Morgan Kaufmann Publishers.]
If: 1) The analysis error (in percent) that is
tolerable is less than 5, and
2) The non-dimensional stress of the
substructure is greater than .5, and
3) The number of cycles the loading is to be
applied is greater than 10000
Then: It is definite (1.0) that fatigue is a
phenomenon ...
A rule like this one, but without antecedent clause (3),
was included. The bad rule subsumed this rule and two
others (stress > .7, cycles > 1000; stress > .9, cycles > 100)
with the same conclusion.