Space Junk: collision imminent
Even with sophisticated tracking and modeling mechanisms, two large satellites managed to crash into each-other. On the one hand it's surprising – even the experts all say "unexpected" and "extremely unlikely". On the other hand, there's an awful lot of junk up there. Tools like JTRACK can give you an idea of just how much stuff there is up there. Even just looking at the working satellites alone, it's an awful lot of stuff to keep track of. Add-in the space junk and dead satellites, and amount of stuff to track becomes pretty overwhelming.