Question: What's an example of something that was a problem for classical physics?
About the article "Physics", section "Modern physics".

Candidate passages:
Three-body problem has two distinguishable meanings in physics and classical mechanics:  In its traditional sense, the three-body problem is the problem of taking an initial set of data that specifies the positions, masses and velocities of three bodies for some particular point in time and then determining the motions of the three bodies, in accordance with the laws of classical mechanics (Newton's laws of motion and of universal gravitation).
In an extended modern sense, a three-body problem is a class of problems in classical or quantum mechanics that model the motion of three particles.
Historically, the first specific three-body problem to receive extended study was the one involving the Moon, the Earth and the Sun. 
Modern physics began in the early 20th century with the work of Max Planck in quantum theory and Albert Einstein's theory of relativity.
Both of these theories came about due to inaccuracies in classical mechanics in certain situations.
Classical mechanics predicted a varying speed of light, which could not be resolved with the constant speed predicted by Maxwell's equations of electromagnetism; this discrepancy was corrected by Einstein's theory of special relativity, which replaced classical mechanics for fast-moving bodies and allowed for a constant speed of light.
Black body radiation provided another problem for classical physics, which was corrected when Planck proposed that light comes in individual packets known as photons; this, along with the photoelectric effect and a complete theory predicting discrete energy levels of electron orbitals, led to the theory of quantum mechanics taking over from classical physics at very small scales.
Quantum mechanics would come to be pioneered by Werner Heisenberg, Erwin Schrödinger and Paul Dirac.
From this early work, and work in related fields, the Standard Model of particle physics was derived.
Following the discovery of a particle with properties consistent with the Higgs boson at CERN in 2012, all fundamental particles predicted by the standard model, and no others, appear to exist; however physics beyond the Standard Model, with theories such as supersymmetry, is an active area of research.
Quantum mechanics and classical physics.
Day-to-day physics.
Since the beginning of the 20th century, quantum mechanics—the physics of the extremely small—has revealed previously concealed aspects of events.
Before that, Newtonian physics—the physics of everyday life—dominated.
Taken in isolation (rather than as an approximation to quantum mechanics), Newtonian physics depicts a universe in which objects move in perfectly determined ways.
At the scale where humans exist and interact with the universe, Newtonian mechanics remain useful, and make relatively accurate predictions (e.g.
calculating the trajectory of a bullet).
But whereas in theory, absolute knowledge of the forces accelerating a bullet would produce an absolutely accurate prediction of its path, modern quantum mechanics casts reasonable doubt on this main thesis of determinism.
Relevant is the fact that certainty is never absolute in practice (and not just because of David Hume's problem of induction).
The equations of Newtonian mechanics can exhibit sensitive dependence on initial conditions.
This is an example of the butterfly effect, which is one of the subjects of chaos theory.
The idea is that something even as small as a butterfly could cause a chain reaction leading to a hurricane years later.
Consequently, even a very small error in knowledge of initial conditions can result in arbitrarily large deviations from predicted behavior.
Chaos theory thus explains why it may be practically impossible to predict real life, whether determinism is true or false.
On the other hand, the issue may not be so much about human abilities to "predict" or attain "certainty" as much as it is the nature of reality itself.
For that, a closer, scientific look at nature is necessary. 
Applied physics is a general term for physics research which is intended for a particular use.
An applied physics curriculum usually contains a few classes in an applied discipline, like geology or electrical engineering.
It usually differs from engineering in that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem.
The approach is similar to that of applied mathematics.
Applied physicists can also be interested in the use of physics for scientific research.
For instance, people working on accelerator physics might seek to build better particle detectors for research in theoretical physics.
Physics is used heavily in engineering.
For example, statics, a subfield of mechanics, is used in the building of bridges and other static structures.
The understanding and use of acoustics results in sound control and better concert halls; similarly, the use of optics creates better optical devices.
An understanding of physics makes for more realistic flight simulators, video games, and movies, and is often critical in forensic investigations.
With the standard consensus that the laws of physics are universal and do not change with time, physics can be used to study things that would ordinarily be mired in uncertainty.
For example, in the study of the origin of the earth, one can reasonably model earth's mass, temperature, and rate of rotation, as a function of time allowing one to extrapolate forward and backward in time and so predict prior and future conditions.
It also allows for simulations in engineering which drastically speed up the development of a new technology.
But there is also considerable interdisciplinarity in the physicist's methods and so many other important fields are influenced by physics, e.g.
the fields of econophysics and sociophysics. 
The semi-classical limit.
What is the semiclassical limit?The classical limit or correspondence limit is the ability of a physical theory to approximate or "recover" classical mechanics when considered over special values of its parameters.
The classical limit is used with physical theories that predict non-classical behavior.
In physics, the correspondence principle states that the behavior of systems described by the theory of quantum mechanics (or by the old quantum theory) reproduces classical physics in the limit of large quantum numbers.
In other words, it says that for large orbits and for large energies, quantum calculations must agree with classical calculations.
The principle was formulated by Niels Bohr in 1920, though he had previously made use of it as early as 1913 in developing his model of the atom.
There are two basic requirements in establishing the semi-classical limit of any quantum theory:i) reproduction of the Poisson brackets (of the diffeomorphism constraints in the case of general relativity).
This is extremely important because, as noted above, the Poisson bracket algebra formed between the (smeared) constraints themselves completely determines the classical theory.
This is analogous to establishing Ehrenfest's theorem;ii) the specification of a complete set of classical observables whose corresponding operators when acted on by appropriate semi-classical states reproduce the same classical variables with small quantum corrections (a subtle point is that states that are semi-classical for one class of observables may not be semi-classical for a different class of observables).
This may be easily done, for example, in ordinary quantum mechanics for a particle but in general relativity this becomes a highly non-trivial problem as we will see below.