Functional Fixedness

I arrived home in the dark yesterday and accidentally put the wrong key in the lock. I took it out, used the right key, and wandered inside. However, once I was inside, I realised that the 'wrong' key had happily gone in. So, uh, yeah, turns out that six months ago when I looked at the keys, I just assumed the two new shaped keys were for the sliding door, and the old looking keys were for the front door. But, actually, like, the sliding door was keyed to fit the front door. D'oh. On the other hand, now I don't need to get another spare key cut, but I have been carrying two of the same key round for the last six months...