  In this paper we study the algebraic structure of error formulas for ideal
interpolation. We introduce the so-called "normal" error formulas and prove
that the lexicographic order reduced Gr\"obner basis admits such a formula for
all ideal interpolation. This formula is a generalization of the "good" error
formula proposed by Carl de Boor. Finally, we discuss a Shekhtman's example and
give an explicit form of "normal" error formula for this example.
