Un-used Re-factorings
Before:
foo(big + wide + long + expression, blah);
Intended:
expr = big + wide + long + expression;
foo(expr, blah);
Mistake:
expr = big + wide + long + expression;
foo(big + wide + long + expression, blah);
Impact: Initial code may pass testing, but future additions to one expression copy or the other is out-of-sync with the other, resulting in the change not showing up or misleading code.
Possible solution: A code analysis tool that warns of un-referenced variables.