  The paper examines local approximation errors of finite difference schemes in
electromagnetic analysis. Despite a long history of the subject, several
accuracy-related issues have been overlooked and/or remain controversial. For
example, conflicting claims have been made in the literature about the order of
Yee-like schemes in the vicinity of slanted or curved material interfaces. Two
novel practical methods for comparison of local accuracy of difference schemes
are proposed: one makes use of Trefftz test matrices, and the other one relies
on a new measure of approximation accuracy in scheme-exact Trefftz subspaces.
One particular conclusion is that a loss of accuracy for Yee-like schemes at
slanted material boundaries is unavoidable.
