A common argument among Christian apologists is that the gospels are an example of “human testimony,” and can be trusted to be accurate. This is because, after all, people can be trusted to be honest, right? Wrong.
The facts are quite different. The gospels were written by anonymous persons long after the alleged events that they describe (assuming they happened at all).  What follows are the approximate dates when the gospels were likely written:
These large gaps of time would easily allow many changes by well-meaning (or not so well-meaning) scribes to alter a text to make it say something it originally did not say. In addition, human memory is known to be fairly unreliable, and the period of time it takes for stories to become distorted, error ridden, and polluted with mythical elements are mere days or weeks. Sixty to eighty-five years is more than enough time for errors to creep into any stories that were being spread about Jesus or any other event.  
However, let us argue that these writers had excellent memories and could have remembered with some detail the events they write about. How plausible is this theory? Well, it’s not likely for the simple fact that a person’s life span during Jesus’ life time was approximately 46 years old. To quote Richard Carrier,
In the ancient world, the average life expectancy (for anyone who survived to age 15) was 46 years, while fewer than 1 in 20 would live to 70, and fewer than 1 in 200 would live to 85. Any witness, who survived the war and was at least fifteen years old by 35 A.D. (and thus could recall events of previous years with any kind of reliability), would probably be dead before 75 A.D. (having only a 34% chance of survival, even without an intervening war and persecution), and would almost certainly be dead by 100 A.D. (with only a 1.5% chance of survival, and that’s again without an intervening war and persecution, which would have reduced the probability of survival a great deal more). […] Likewise, Josephus himself says 20 years is enough time for witnesses to no longer be available to rebut a story (Life 360; cf. Jewish War 1.15 & Against Apion 1.55). 
Given the fact that the earliest biblical manuscripts are no earlier than about sixty years after the events in question it’s highly unlikely that any eyewitnesses that were alive during Jesus’ alleged crucifixion and resurrection were still alive when the gospels were written. Therefore, there are no first-hand reports of these events, casting much doubt upon their veracity.
Eyewitness testimony is often considered in scholarly circles to be one of the least reliable forms of evidence for a claim and the bible is such a poor example of this form of argument that there doesn’t appear to be any actual witnesses in the gospels in the first place.
1. http://www.earlychristianwritings.com/ – accessed 1-10-12
2. Memory Distortion: How Minds, Brains, and Societies Reconstruct the Past, Edited by Daniel L. Schacter, Harvard University Press, 1997
3. Jesus is Dead, by Robert M. Price, American Atheist Press, 2007; 36