I once spent an afternoon tracking down a "bug" as to why sales tax wasn't being calculated on LedgerSMB only to find out I had set the tax rate to 0 in the tax interface.... Ok, it was working as intended. I felt pretty sheepish too.
The problem in my case is that sales tax calculation easily qualifies as a big deal and so any sense that it's not working raises all sorts of alarm bells. In addition to the immediate questions of "are production versions affected? If so what do we tell customers?"
Also taxes with a rate of 0 are ignored specifically because sometimes sales tax structures change (as with HST consolidation in Canada) and consequently old taxes need to be retired.....
While I am sympathetic to this argument, I would say that is not always the case. Some configuration issues are usually required and when something is set up for a specific case, and it behaves for that case, and the user simply forgot that this is what they did, then it's a bug only in the storage retrival routines of the user's own memory.
