Isaiah 14:12 (King James Bible) how art thou fallen from heaven
These two examples show the tendency in the Book of Mormon text to replace the archaic art with are. In the first example, Oliver Cowdery initially wrote are, but then virtually immediately he corrected it to art (there is no change in the level of ink flow). In the second example, the typesetter for the 1888 LDS edition accidentally set are. Such accidental errors are readily corrected and generally do not persist in the history of the text.
Also in this chapter we have a couple of examples of the archaic hast being momentarily replaced by has:
Summary: There is a tendency in the text for the archaic verb forms associated with thou to be replaced by the more general verb forms (art by are and hast by has); the critical text will retain the original archaic forms, providing they are supported by the earliest textual sources.