S_N2 reaction leads to complete inversion in configuration but S_N1 reactions may lead to racemization. However, evidences indicate that all the molecules of particular substrates do not necessarily react by a single mechanism. After all, the intention of the molecules is very clear; they want to follow the low energy path. If both paths have nearly the same energy barrier, they follow a mixed mechanism. For example, secondary alkyl halides follow a mixed mechanism; say 60% molecules react by S_N2 and rest 40% by S_N1. Stereochemical features, as mentioned above, are noticeable only when the reaction is carried out on a particular enantiomer. Even the reaction centres, which are parts of suitably substituted rings, has a role to play.