Among some orthodox and Hasidim, matzoh balls are not kosher for Passover because the matzoh is in contact with water for more than 18 minutes (and hence considered to have "leavened"). There are matzoh factories in Boro Park and Williamsburg where the making of matzoh is actually timed so as not to exceed the Talmudic maximum.
In much wider orthodox traditions, eggs can't be mixed with matzoh for the first two days of Passover.
(But I'd ignore all of that, and follow Marilyn's advice, at least if you are of the floater persuasion. My mother is a sinker, and my wife is appalled!
)