The weakest link

Unfortunately, people sometimes do not seriously think about which is really the weakest link in a security system. And there is also a more subtle issue: strengthening a link may cause the attacker to change his strategy, with undesirable consequences. I remember a story about some car anti-theft devices being "too" secure in making hot-wiring difficult, so that they actually caused an increase in carjackings. It is probably better (as well as much safer) to discover that one's car has been stolen when returning to the car park...

(comic from xkcd)