Reactions between enantiomers in a racemic mixture
If a chiral molecule (an enantiomer) can react with itself, dimerize for example, to form a new compound, then formation of this new compound would contaminate the racemic mixture.

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Perhaps more importantly is interaction of the racemic mixture with itself in the solid state.  Just as the interaction between a right hand and left hand is different than the interaction between two right hands, the interaction between the R form and S form of the enantiomer will be different than the interaction between two R (or two S) forms.
In liquids and gases, molecular encounters are fleeting and cannot be preserved or &quot;locked in&quot; for significant periods of time as they can be in a solid. In other words, any favorable, lower energy molecular encounters are quickly broken apart by the energetically larger effects of thermal motion.  However, when the racemic mixture is in a solid phase, if the interaction between R and R (or S and S) is lower in energy then the interaction between R and S, then the R and S enantiomers will tend to form separate crystals and a mixture of pure R and pure S crystals will result.  Such mixtures are often termed &quot;conglomerates&quot;.  On the other hand if the interaction between R and S is more favorable (lower in energy), then a racemic compound or racemate is formed where each crystal contains R-S pairs.
A conglomerate, being a simple mixture, will show a melting point depression compared to the pure enantiomer.  A racemic compound may have a melting point higher or lower than the pure enantiomer.
Further reading on racemic conglomerates and racemic compounds can be found here and here.