Originally Posted By: latishya
In the 1630s, the word juggernaut is first attested, which represents the Hindi word jagannāth, meaning "lord of the universe".


This example of an unpronounced R being put in a word not previously known in English is consistent with The Pook's conjecture that the second R in sherbert was a result of non-rhoticism. The earliest citation for the sherbert spelling in the B&M OED is from 1675: We were severall times treated with sherbert of lemmons. Thanks, latishya.

Incidentally, the OED also lists variations with the first syllable spelled ser-, cer-, and sar-. I'm still wondering about the initial sh.