In the Merriam-Webster Unabridged (2013), under the definition of the verb err one finds the following information:
Usage Discussion of ERR
The sound of the letter r often colors a preceding vowel in English, so that the originally distinct vowels of curt, word, bird, and were are now pronounced the same. Originally err and error had the same first vowel, but over time err developed the pronunciation \ˈər\ as well. Commentators have expressed a visceral dislike for the original pronunciation \ˈer\; perhaps they believe that once usage has established a new pronunciation for a word there can be no going back. By this reasoning, though, we should embrace the once established innovative pronunciations of gold \ˈgüld\ and Rome \ˈrüm\ (as seen in Shakespeare’s pun on Rome and room in Julius Caesar I.ii.156). For these two words the English language has returned to the older forms, and no sound reason prevents us from accepting again the \ˈer\ pronunciation of err, which is today also the more common variant in American speech.
In defending the modern mispronunciation of this word, what the contributors to this dictionary have failed to take into account are the markedness relations that support the traditional norm. The verb is the marked grammatical category (due to its obligatorily explicit reference to time, lacking in the nominal categories of substantive and adjective). Hence we should expect the vowel implemented in the verb to be the marked vowel (here schwa), as opposed to the unmarked /e/ of the noun error and the adjective errant. And that is exactly what we get.
This particular distribution of vowels, as between the two opposed grammatical categories, is what is semeiotically called an ICON OF RELATION, i. e., a DIAGRAM. Language over time (teleologically) always tends toward a patent diagrammatization of sound and sense, as the case of err demonstrates.