It's easy to sneer at Harold Camping for his all-out Doomsday prediction, which had the end of the world pegged at May 21, 2011, 1800hrs local time -- especially since as I write this, it it now May 22, 2011, 0700hrs Pacific Time, and the predicted opening of the earth hasn't happened. But there are a couple of key lessons here.
First, discounting the possibility that this was just a hokey publicity stunt to bring more people and dollars to his ministry (and we know that evangelists have NEVER done that before!), let's assume that the basic motivation was the same thing that motivates any Christian: a desire to see as many people Saved as possible on the principle that friends don't let friends go to hell.
Second, let's assume that Harold Camping was given a strong leading from God to warn people in general that the signs that Jesus foretold as preceding His return are building up (and they are). It's entirely possible that's what happened, but can happen to any of us, pride got mixed in with the anointing so that he believed he'd been shown something no one else could see. He who has never been misguided can cast the first sneer.
Scripture tells us to "touch not [God's] anointed, and to My prophets do no harm." (1 Chr. 16:22). (That's the reminder (1 Sam. 26:11) that stopped David in his tracks when he had Saul dead to rights during the civil war.) In other words, we don't judge or ridicule someone who may have heard an instruction from God but was mistaken in the way he carried it out.
Third, Jesus says, "... of that day and hour knoweth no man, no, not the angels of heaven, but My Father only." (Matt. 24:36). While there are plenty of invitations throughout the Bible to delve deeper on particular topics and search out mysteries of God and His plans, there is no place that suggests we'll be able to "do the math" for ourselves and devine that "day and hour"; and Jesus specifically tells us not to bother.
There's a good reason for that. If you know where the finish line is, you know where you can let up and coast the rest of the way. Jesus doesn't call us to "coast". We are called to run the entire race (1 Cor. 9:24), which means we don't know where the finish line is. We have to be prepared constantly for Jesus' return, drawing ever closer to Him and letting God order our steps.
In Matthew 24, Jesus warns about false Christs (not that I'm suggesting Camping is one) and those who say, "He is in the desert" or "He is in the secret chamber." Jesus tells us not to get suckered in, because that's not where He will turn up. Rather, His return will be like the daybreak from east to west: gradual, unstoppable, constantly lightening our sky at the turning of the day (Matt. 24:27).
And this leads to what may be the biggest lesson of all from Harold Camping's prophecy: know your Scripture. It's not like that's a big freaking secret: God has given us His Word, complete and unabridged with nothing further to add, so that we can know exactly what He plans and know exactly what His voice sounds like when we hear something in the Spirit. People who know their Scripture know that any pinpointing of Doomsday is all wet, simply because of what Jesus tells us in Matthew 24.
People who don't know Scripture (a) sneer at evangelists who warn of judgment day, (b) fall for the prophecy and are then disillusioned when it doesn't come to pass or (c) wind up gathered around a pot of Kool-Aid in Guyana. They also don't know that the signs that Jesus did foretell are happening, and that means we have to Get Ready: draw closer to God so He will draw closer to you. Let Him work on you to bring you more into the image He has of you. Get to know His Word and what it really says, so you're not caught out -- or left behind.