Sherbert or Sherbet?

A few days ago I brought up the Mackinaw vs. Mackinack question. I reached the conclusion that most customers were really pronouncing the word the correct way and I was the one that was wrong. Congratulations, America.

But when it comes to sherbet all bets are off. Most people who order sherbet-flavored ice cream pronounce it "sherbert," as I did when I was a kid. This mispronunciation knows no age limits; people young or old will order "sherbert." I don't really have any problems with it, as the corruption is so common as to be accepted, but I really want to know where this mangled word came from. Is it because it gives the word a kind of symmetry? Is it because of the name "Herbert?" Did this form become commonplace after the Supreme Court decision Sherbert v. Verner?

(Actually, the Supreme Court case does give me a clue: if Sherbert is a surname, probably German, did the corruption stem from people confusing the flavor with someone they knew with that name?)

Google hasn't turned up a study on this. Maybe this is because this is something you can only track through speech and not the written word because no one ever writes the word as "sherbert." After all, spell check puts that red line through it. But someone must have pinpointed its first appearance in language.