The coexistence of the indefinite articles a and an has been a lasting source of confusion in speech, inevitably leading to the creation of new words. Throughout history, these mixups have made their way into writing, thus allowing us to retroactively track their progress.
• "a napron" led to "an apron" (1400-1600)
• "a nadder" led to "an adder" (1300-1500)
• "a nauger" led to "an auger" (1300-1500)
Almost simultaneously, the letter f was in some words undergoing a transformation that would lead it down many paths, from v to w to u, each with uncertain destinations. The Old English hafoc would sprout multiple forms in the 12th to 14th centuries, from hauek to havek to hauck, before ultimately settling on hauk in the 15th-17th centuries, leading to its present form, hawk.
A similar story occurred with the previously-mentioned auger, which in Old English was rendered as the compound nafu-gár - literally, "nave-borer", from nafu ‘nave’ (of a wheel) + gár piercer, borer, spear. The Old English -af- passed through -av- to -aw- and -au-. In combination with the dropped n through confusion of "a nauger" with "an auger", we have an example of two changes working together to evolve a word away from its original form. (Illustration: Study of a man with an auger, for The Seven Sorrows of the Virgin, Albrecht Dürer, ca 1496)
This is even more true in the case of the Old English efeta. Over half a millennium, this ancient word would evolve into two different words with the same meaning - one quite similar to the Old English form, and one very different. As efeta underwent the ef- transformation into ev-, eu-, and ew-, some people were simultaneously mixing the spoken word with its preceding particle, an. Eventually, forms such as evete would shift back to the original usage of f in the modern word eft, but by then the word had also been corrupted into forms such as euete and ewt, which in combination with the indefinite particle an, led an ewt to become a newt.
Source: various entries. Oxford English Dictionary. Second Edition, 1989. Retrieved 2007-06-22.