The important thing to remember is that as sure you are that they're wrong, they're sure that they're right. In general I find that asking them to explain why

The important thing to remember is that as sure you are that they're wrong, they're sure that they're right. In general I find that asking them to explain why they think this is causing the behaviour you're seeing they can usually work out they're wrong/convince you they're right. Of course this only works if they don't scream "I've found the bug" every time you step forward.