Focusing past infinity?

[Unimaxium]

This is a question I've been wondering about for a while, so I decided to go ahead and ask it.

I've noticed that on several of my lenses (for both my Canon and my K1000... see my profile), the focusing ring allows me to focus slightly past infinity, i.e. when the lens is focused at the farthest possible object, I can still turn the ring a little farther and make it out-of-focus again. I was wondering what function this extra focus serves? Theoretically there should be nothing it can focus on past infinity, so why can I focus to it? Wouldn't it be nice and easier if it could just stop at infinity so I knew exactly when it's focused on a distant object?

So, any ideas?

 

[photobug]

Some lenses with plastic barrels can change their physical length as the temperture changes. The "slop" in the infinity focus mark allows for that.

Then again, it may just need to be adjusted.

 

[DocFrankenstein]

An imperfection in the lens design?
Or is that the extra adjustment you have to make when shooting IR? I forgot...

 

[jadin]

All my lenses do the same thing, so I don't think it's an imperfection.

 

[Hertz van Rental]

It's probably there to allow for slight inaccuracies in manufacture. You only need to be out by a fraction of a mil for a lens to not focus properly.
There is also some doubt as to exactly where infinity is so it's hard to focus on a concept