Re: Experienced tester.
Have I got a great example of that one. The social security managing software that the company I work for makes has some very interesting 'features'. For example, the client in a dossier and his partner are stored in 2 separate fields in said dossier.
When we make a new dossier, we check the existing ones to see if there are any incompatible dossiers around and still active. If there are, we show an error. This check of course only checked for clients, not partners.
So you could give client A and partner B their social security check and then give client B and partner A their social security check as well, paying them twice because partner B didn't already have a dossier as far as the system was concerned.
It only took a year to get it accepted as a bug and fixed, because of course the user and their processes wouldn't allow that situation to happen in reality.