Though we grumble about it, most native English speakers have just accepted that sometimes the language doesn't make a whole lot of sense, contenting ourselves to memorise an elaborate series of tricks and sayings to help us keep things straight. But it seems one question has been plaguing people for long enough that someone had to research it: why do people add an extra "r" that doesn't exist when pronouncing the word "sherbet"?
The short answer? No one really knows. The professionals certainly don't encourage use of the second "R". Says Smithsonian:
"Despite widespread use and general public acceptance that second 'R' is virtually non-existent in the frozen food aisles of supermarkets, or in [US] ice cream chains like Dairy Queen and Baskin-Robbins ... In fact, a spokesperson for Baskin-Robbins assured me, "The brand has spelled 'sherbet' with just one 'r' since its founding [in 1945]." This, she says, is consistent among their franchises nationwide."
One theory is the expectation of rhyme. Because the first syllable ends in -er, we want the second one to do the same. Smithsonian interviewed Michael Adams, English language historian and Indiana University-Bloomington provost professor, who said:
"When I'm reading aloud to my children I sometimes unconsciously repeat sounds in syllables or words that closely resemble each other, and then I re-read the phrase. Sherbet is begging to be pronounced Herbert on this 'principle.' It isn't a type of systematic change in language," he says, "but a lexical change," meaning that it's not a particular environment that's influencing our pronunciation but more a change in preference.
Another possibility is that the mispronunciation is so widespread that it's now become self-reinforcing. Or this may all be the fault of the Big Band era, when composer Ben Homer released the 1939 hit, "Shoot the Sherbet to me Herbert."
The song pronounces it as "sherbert" despite using the correct spelling in the title, and may have had a significant hand in introducing the satisfying-but-incorrect pronunciation into the mainstream.
This mystery may never be truly solved, but the important facts still remain: Linguistic evolution is weird, prescriptivism is boring and frozen desserts are delicious.