  The main result of the paper is that the Lifshits--Krein trace formula cannot
be generalized to the case of functions of noncommuting self-adjoint operators.
To prove this, we show that for pairs $(A_1,B_1)$ and $(A_2,B_2)$ of bounded
self-adjoint operators with trace class differences $A_2-A_1$ and $B_2-B_1$, it
is impossible to estimate the modulus of the trace of the difference
$f(A_2,B_2)-f(A_1,B_1)$ in terms of the norm of $f$ in the Lipschitz class.
