Not a true contradiction per se, but this still seemed like the most appropriate place to bring this up.
Let's say for the sake of argument that the stories are true, granting that Jesus magically multiplied the food, and the disciples were just as surprised the second time around as the first. This still does nothing to address exactly how they were eating those fish.
First of all, where did these fish come from? Who carries fish around with them for a snack? Bread I can see, but fish???
The stories make it sound like they were whole fish. How on earth would they prepare it? Were there also people carrying filet knives with them, cleaning the fish as they magically multiplied? Did they build a fire to cook it all? Or were people just chomping into raw fish?
The author could have used any fruit, vegetable, or liquid, and it would have at least given the stories some semblance of believability. As they are written, they are ludicrous entirely apart from the magical bits and bad character building.