here's a word that has undergone recent enantiodromia: nonplussed

In standard use, nonplussed means 'surprised and confused'. In North American English , a new use has developed in recent years, meaning 'unperturbed'--more or less the opposite of its traditional meaning. This new use probably arose on the assumption that non- was the normal negative prefix and must therefore have a negative meaning. Although the use is common, it is not yet considered standard.
- New Oxford American Dict.