Can anyone explain to me the law of reciprocity and under which circumstances it might fail? 
Put simply:
In photography and holography, reciprocity refers to the inverse relationship between the intensity and duration of light that determines exposure of light-sensitive material. Within a normal exposure range for film stock, for example, the reciprocity law states that the film reponse will be determined by exposure = intensity × time. Therefore, the same response (for example, the optical density of the developed film) can result from reducing duration and increasing light intensity, and vice versa.
The reciprocal relationship is assumed in most sensitometry, for example when measuring a Hurter and Driffield curve (optical density versus logarithm of total exposure) for a photographic emulsion. Total exposure of the film or sensor, the product of focal-plane illuminance times exposure time, is measured in lux seconds.
For most photographic materials, reciprocity is valid with good accuracy over a range of values of exposure duration, but becomes increasingly inaccurate as we depart from this range: reciprocity failure, reciprocity law failure, or Schwarzschild effect.[1] As the light level decreases out of the reciprocity range, the increase in duration required to produce an exposure becomes higher than the formula states; for instance, at half of the light required for a normal exposure, the duration must be more than doubled for the same result. Multipliers used to correct for this effect are called reciprocity factors (see model below).
During very long exposures, film responds less than usual. Light can be considered to be a stream of discrete photons, and a light-sensitive emulsion is composed of discrete light-sensitive grains, usually silver halide crystals. Each grain must absorb a certain number of photons in order for the light-driven reaction to occur and the latent image to form. In particular, if the surface of the silver halide crystal has a cluster of approximately four or more reduced silver atoms, resulting from absorption of a sufficient number of photons (usually a few dozen photons are required), it is rendered developable. At low light levels, i.e. few photons per unit time, photons impinge upon each grain relatively infrequently; if the four photons required arrive over a long enough interval, the partial change due to the first one or two are not stable enough to survive before enough photons arrive to make a permanent latent image center.
This breakdown in the linear relationship between aperture and shutter speed is known as reciprocity failure. Each different film "emulsion" has a different response to long exposure. Some films are very susceptible to reciprocity failure, and others much less so. Some films that are very light sensitive at normal illumination levels and normal exposure times lose much of their sensitivity at long exposure times, becoming effectively "slow" films for long exposures. Conversely some films that are "slow" under normal exposure duration retain their light sensitivity better at long exposures. Compared at very long exposure times, Kodak's T-Max 100 speed film is faster than the nominally four-times faster Tri-X 400. Most film manufacturers publish reciprocity corrections.
For example, for a given film, if a light meter indicates a required EV of 5 and the photographer sets the aperture to f/11, then ordinarily a 4 second exposure would be required; a reciprocity correction factor of 1.5 would require the exposure to be extended to 6 seconds for the same result. Reciprocity failure generally becomes significant at exposures of longer than about 1 sec and below about 1 ms for film, and above 30 sec for paper.
Reciprocity also breaks down at extremely high levels of illumination with very short exposures. This is concern for scientific and technical photography, but rarely to general photographers, as exposures significantly shorter than a millisecond are only required for subjects such as explosions and particle physics experiments, or when taking high-speed motion pictures with very high shutter speeds (1/10,000 sec or less).
Innit.
