Smidt

Another version of the German last name "Schmidt", only those who have the name "Smidt" almost always have to spell it out.

Also, the "D" is silent, but never fails to confuse people.

Person A: "Name?"
Me: "Smidt."
Person A: "Schmidt. ...Nope, can't find your reservation."
Me: "...That's Smidt, S M I D T."
Person A: "...S M I D T? ...Oh, here it is."

...or...

"Your last name is Smiddit?"
"...No."