Metathesis is a phonological process by which two circant (adjacent or near-adjacent) phones (or sounds) switch places. Despite its strange definition, metathesis is a fairly common phenomenon in historical sound changes observed in linguistics.
A historical example is the development of the english word "wolf" from the Indo-European root LUP:
LUP > ULP > ULF > WOLF
The first sound change is metathesis of U and L, the second is labial spirantization (occurred in Germanic languages, compare with Lat. pater to Germ. vater), and the last change U > WO was common in the shift from Old German to Old English. Interestingly, the metathesis here is thought to be motivated by superstitions that naming a wolf by its "true name" LUP would bring bad luck. It is an example of taboo language.
Examples in modern English are exhibited in both infantile and urban English. For example, "aminal" for "animal," "enemy" for "emeny," "pasghetti" for "spaghetti," and "aks" for "ask."