Is there a term for a flaw in logic?

Depends on the point of view, doesn't it?

When someone finds a logic error in my code, I call it "a small misunderstanding" and blame it on marketing's requirements doc.

When I find a logic error in somebody else's code, I call it "a total fsck()ing disaster!" and blame it on management's hiring policies.

In general, though, "bug" is not only the right word; it's the only word. You can't go wrong with "bug" in any context having to do with code.

A "logical fallacy" means:

Clearly defined error in reasoning used to support or refute an argument, excluding simple unintended mistakes.

Wikipedia provides a good list of fallacies. You may be able to find a more descriptive term for whatever error you happened across.

Response to addendum: "Fallacy" works fine on its own. Fallacies imply nothing about intention and context will clue people into the implicit "logic" topic. If that doesn't do it for you, here are a few other options:

• irrational
• illogic
• mistake
• accident

If you are looking for a term to use in class or variable names, "flaw" or "error" would probably be fine with some good supporting documentation describing the problem. logicError is certainly legible.

It may seem too easy but

logical flaw

or

logical error

works, to distinguish it from other flaws or errors (like typos or errors of fact).

How about "illogicality"? I just made it up, but then I googled it, and it already existed!

I can't find a specific definition for it, but it's listed as a noun under "illogical" in several dictionaries....So, most basic definition would be "a thing that is illogical."