Mat 21:19b ...and the fig tree withered at once.
Mark 11:20 As they passed by in the morning, they saw the fig tree withered away to its roots.
And you will say that the person who wrote the second account was so unobservant that he did not see the instant withering -- not a very trustworthy witness, eh? In Mark it takes overnight for it to wither, but in Matthew it withers right away.
Mark simply omitted a detail that Matthew included. This is pretty common when two different eyewitnesses describe the same event. One person emphasizes a detail that impressed him and omits others, another emphasizes and omits others, because their vantage point enables them to see some things and prevents them from seeing others. To me, what really piques my suspicion is when two stories are exactly alike, then you know the people rehearsed them to get them perfect. Mark does NOT say that the fig tree took all night to wither; he mentions no duration for the withering whatsoever. You wrongly infer that idea. Mark says only that they passed by the withered tree in the morning...no contradiction.
Bear in mind that this was not like other of Jesus’s miracles where people anticipated it. It was without warning, almost like a whim. There is the distinct possibility that Mark did not actually see the instant withering, but did see the withered tree in the morning. Perhaps he was momentarily occupied. It wouldn’t take much distraction at all to miss an instant withering. So Mark recorded what he actually DID see and not what he did not see.
Here's how I interpret the two passages: Jesus curses the tree, and there is an instant noticeable effect on the tree - it withers. Some of the disciples (Matthew) see the instant withering, and some (Mark) don’t, because it was unexpected. However, they both heard Christ curse the tree. As they pass by in the morning, the tree is even further withered - down to its roots, thoroughly destroyed. That strikes me as a reasonable interpretation. However, I wouldn't be against the tree withering fully right away and Mark simply not noting the instancy in his account because he perhaps was not an eyewitness to it. After all, the important thing to anyone is not really how fast it withered, but that it withered at all at Christ's words. So...not really impressed with this "contradiction".