Bullet proof code?

I was asked to look at some issue earlier today, and specifically told to "bullet proof" the code so that the error doesn't occur, seeing as we can't reacrate it. I looked at that statement for a while, wondering to myself what exactly is expected of me. What does the phrase "bullet proof code" mean anyway. Surely not code that doesn't have bugs. That's just not possible. Again, helpful information regarding the bug isn't forthcoming. Now I don't know where to start.