I think you mean fossils that appear in geological layers where they wouldn't normally be expected. This could happen in a few ways.
First, plate tectonics. As plates move, and diverge, and converge, several layers can crash together, creating a new land formation, and jumbling things up.
Secondly, glaciers. Fossils frozen in glaciers that receded at the end of the Ice Age could be deposited on top of a layer, making it seem as though it was a relic, when it was just a matter of the bones being moved from one place to another via the glacier.
Thirdly, water. Animals tend to stay near water sources. As a result, many animals also die, and become fossilized, near water sources. A fossilized skeleton (or even the unfossilized remains) drift downstream, are deposited in a different area of rock, covered up, and looks like it was fossilized there all along.
Forgive me if this is a rather crude analysis, as I'm not a geologist, but I believe those are a few ways that such things can occur.
Some believers might well suggest that such occurrences are evidence for the Great Flood. This is simply not the case. Evidence for the great flood would be in the form of fossil remains of every possible living thing, all in the same layer corresponding to roughly 4,000 years ago. Then, there would be no fossil record at all for quite some time afterwards, at least in most places. Mt. Ararat would be the epicenter for any fossil remains from anytime after that. Species would be distributed from that point outwards, with the highest concentration of species near the epicenter, and gradually decreasing as one goes outwards from there. However, none of this is the case, which is of course excellent evidence against the Great Flood.