How does a smart-key go bad?

My daughter could not get her the van to start.  It turned over but would not fire.  She assumed she had run out of gas, so I added some and found that it started up rather easily.  The next morning she had trouble again, but it started perfectly for me when I tried it.  It took a little investigating, but we eventually determined that it was her key that was the problem.  We were both using "smart-keys" that have some sort of electronic chip in them to allows the van to start.  But how did her key go bad?  I still do not know.