The incorrect statement is that enantiomers are superimposable mirror images. By definition, the stereoisomers related to each other as non-superimposable mirror images are called enantiomers . The other statements are correct:

1. A racemic mixture contains equal proportions of two enantiomers, so the optical rotation of one isomer is cancelled by the other, resulting in zero optical rotation .
2. In an $\text{S}_\text{N}1$ reaction, the intermediate carbocation is planar, allowing nucleophilic attack from either face, which results in racemisation (a 1:1 mixture of enantiomers) .
3. An $\text{S}_\text{N}2$ reaction proceeds with the nucleophile attacking from the side opposite to the leaving group, resulting in an inversion of configuration .