Prophecy has been brought up numerous times. Prophecy is a popular topic amongst the religious, as that which has been seen to be fulfilled is an important claim to legitimacy. While I appreciate and admire the sentiment of Christianity, and many of the moral lessons of the Bible, reliance on prophecy is extremely problematic.
The Bible predicted many things that were supposed to have happened in the past or have not yet happened. That is, there are plenty of examples of unfulfilled prophecy.
The Ezekiel prophecy of the destruction of Tyre by Nebuchadnezzar is a dubious one. Scripture says fairly blatantly that Tyre would be completely destroyed. It was not, but let's say that this one can be considered partially fulfilled.
Ezekiel describes the utter destruction of Egypt by God and turned into a desolate waste for 40 years. Perhaps this is still going to happen, because it hasn't happened yet, and I doubt this prophecy was intended to take that long. Additionally, Nebuchadnezzar was supposed to have attacked and destroyed Egypt and filled the land with the bodies of the slain. Neb's only attempt at invading Egypt was not a success.
Both Ezekiel and Isaiah predicted that the Nile would dry up. Isaiah gives a pretty clear time line, not something that may still be fulfilled, but to have happened during pagan Egypt. The Nile is not nor has it ever been dry.
The Isaiah prophecy that Egypt would speak the language of Canaan and adopt Judaism. Perhaps we can loosely interpret that Isaiah is referring to Hebrew in general. Otherwise this prophecy is a virtual impossibility since the Canaanite languages are long dead. So, I guess perhaps this prophecy could still come to pass and Egypt will speak Hebrew and adopt Judaism? Call me a skeptic on this one.
The Ezekiel prophecy that Israel will live in peace with it's neighbors must be on that will still come to pass, as the entire history of Israel goes in direct contrast to it. No specific time line is given, so I guess time will tell.
There are other examples, just as I am sure there are theists who can point out examples of fulfilled prophecies. Additionally, this is not meant to ridicule Christians, dissuade faith, or claim that Christianity is full of it. I do have serious issues with prophecy, however. Prophecy is often touted as a testament to Biblical inerrancy, yet we are asked to disregard clearly unfulfilled prophecies, or at least not count them against this idea of inerrancy.
What value I find in the Bible comes not from considering it the inerrant Word of God, so I'm not under any burden to prove anything. Those that make this claim, however, are under such an obligation.