This leads to wear on the card; it gets scanned several times per day. The cards aren't made as tough to this as they probably should be; they're prone to fading down the middle (where they contact most with the scanner). I saw someone's break in the scanner at Bear's Den a few days ago.
The middle fading usually just results in a half-illegible name: the top half, which is obliterated by a white streak, after about a year. Which is okay, because by that time, you should know your name.
In a half-faded state, however, it can have unusual effects. My photo is not particularly flattering; it contains from slightly above my chin to slightly below the top of my head. The zoom-in was a bit close, as my lack of ears in the image shows. It is actually possible to make out my individual teeth in the image.
Even this short time into the year, the scan-streak is visible on my ID card. It goes, like everybody's, right across the middle. Thing is, that's where my eyebrows are, in the image portion.
I appear to have bleached eyebrows on my ID card. The fading is just enough right now that they're a light shade of "grandmother-head gray." (Not blue, though.) Everything else seems pretty much fine; although it's as prevalent, the corresponding white streak in my hair seems less noticeable somehow.
Or at least less notable.