How accurate is the law of superposition? Recently I started being more interested in the rational details of the sciences (... being often challenged about them by a creationist friend) and one of the creationist 'theories' I've encountered is how the law of superposition - i.e. the claim that layers of rocks form with the top layer being younger than the bottom - can be falsified. They mentioned some creationist experiments where layers formed 'sideways' and used it as one of the arguments to 'prove' that Earth does not have to be millions of years old.

Now I do not want to engage in a theological discussion, but I was wondering how accurate law of superposition is. Most articles I've read refer to it as an axiom of geology - i.e. something that is assumed, rather than proved. At the same time I am aware of the fact that it's a "first level big picture axiom", in that everyone is aware of the fact that under special circumstances the details get a bit foggy, but looking at the whole world it is correct "often enough" to warrant making it an axiom to simplify the theory for beginning geologists.

Thus my question is: In the global scope, how significant are the deviations from law of superposition in the structure of Earth's strata? Would you say less than 1% of the world show any significant deviation from the law?

I apologise for any jargon misunderstandings I may have committed.
The principle does not apply to all materials on earth as observed by us but is primarily concerned with sediments and rocks that form from sediments. Since sedimentation is a process driven by gravity grains settle one on top of others and there is no possibility to change that without adding additional processes. So from this perspective the principle is 100% in situations where it is applicable. 

It is of course not uncommon to find situations where older material overlays younger. In such cases there has been additional forces (natural mind you) in action. It is for example possible for intrusions (magma) to be injected in older solid rocks thus completely changing the age sequence. It is also possible for crustal movement (tectonics) to fold the bedrock and thrust parts of the bedrock to overlay other parts hence creating situations where younger rocks seem to undelie older. It is also possible for sediments to slide or move downslope so that a reverse or at least disturbed age sequence is established. I could continue ad infinitum. 

So many possibilities exist to obtain such cases but always with a logical explanation involving post-sedimentary processes. To correctly interprete observations, one has to have gained full understanding of the geological setting and history as well as physical processes. It is not generally wise to look at an outcrop or cut and create a larger scale interpretation without such background knowledge.