Lambert Beer's law deviations
A compound S follows the Lambert-Beer law if the absorbance $E_\lambda$ at a particular wavelength $\lambda$ is proportional to the concentration $c$ of S.

\[E_\lambda = \left(\frac{I_0}{I} \right) = \epsilon\cdot c \cdot d \]

A "deviation" is observed when the real concentration $c$ is different from the expected/assumed.

This may happen if


S is an acid or a base: 

variation of the pH leads to (de)protonation and forms another species with a completely different absorption spectrum, or at least a different $\epsilon$ at the wavelength under observation

S undergoes a reaction to S' (with different optical properties) at elevated temperatures (or change of pH)
Decrease of temperature or change of pH leads to the formation of a turbid solution from which light is scattered into the detector