To put this in context, I have the following scenario. I am writing a Common Lisp program that works with strings and lists of characters.
In a certain function
foo, the value of a variable
suff is a list of characters. Further on in the code, I forgot that
suff was a list and treated it as a string, calling
(subseq suff 0 1)
I did not notice the mistake, because
subseq works both on strings and lists:
CL-USER> (subseq "abc" 0 1)
CL-USER> (subseq '(#\a #\b #\c) 0 1)
So in the subsequent code I assumed I was working with strings while in fact I was moving around lists of characters.
These wrongly-typed results were finally formatted by the program's output function using
(concatenate 'string .... I was lucky, because
concatenate happily produces a string when given lists of characters as arguments.
I only discovered this mistake when adding more tests (yes, I know, TDD, but that's another topic) and testing
Since my program was running properly - at the time my output function was the only piece of code that was using the corrupted data - I cannot say that the program as a whole contained a bug even though function
foo, taken separately, was buggy.
Is there a special name to describe such an incorrect internal behaviour that does not manifest externally as a bug?