The earliest text for Mosiah 12:14 reads “this man hast lied concerning you” (the reading in 𝓟). The 1830 compositor set the hast, but for the 1837 edition Joseph Smith edited the verb to has; he also edited the following hath in “and he hath prophesied in vain” to has. (For discussion of the rather frequent emendation of hath to has, see under infl tional endings in volume 3.) But in the 1840 edition, the first instance of the emended has was changed to hath. This reading continued in the RLDS textual tradition until the 1908 edition restored the has that Joseph had written in the printer’s manuscript when he edited the text for the 1837 edition.
Most probably, the original text here read hath in both cases (“therefore this man hath lied concerning you and he hath prophesied in vain”), but the first hath was accidentally replaced by hast because of the use of hast in the immediately preceding clause (“and thou O king hast not sinned”). This error could have occurred early in the textual transmission, either as Joseph Smith’s dictation was taken down by the scribe in 𝓞 or as Oliver Cowdery copied from 𝓞 into 𝓟 for this passage. We have evidence that Oliver sometimes replaced hath with hast, even without a preceding hast. In the following example, 𝓞 is not extant, but it probably read hath; Oliver initially copied hast into 𝓟 but then virtually immediately corrected the text to hath (there is no change in the level of ink flow):
Thus there is independent support for Oliver accidentally replacing hath with hast in Mosiah 12:14.
More generally, there has been some tendency for the scribes to accidentally replace the -eth ending with the second person singular -est ending; in each of the following examples there is no nearby occurrence of -est that could have triggered the error:
There is one case, however, where the -est ending apparently extended itself to another verb:
The use of sayest in 𝓞 appears to be the result of anticipating the following believest. See under that passage for discussion.
Thus it should not be surprising that in Mosiah 12:14 Oliver Cowdery might have accidentally replaced hath with hast, especially given the preceding “and thou O king hast not sinned”. For further discussion regarding the mix-up of the two inflectional endings -(e)st and -(e)th, see under 1 Nephi 11:2. For a complete analysis, see under infl tional endings in volume 3.
Summary: Accept in Mosiah 12:14 the 1840 emendation of hast /has to hath (“therefore this man hath lied concerning you”); also restore the hath in the following clause (“and he hath prophesied in vain”).