The authors examine the nodal structure of accurate helium wavefunctions calculated by direct diagonalization of the full six-dimensional problem. It is shown that for fixed interelectronic distance R (or hyperspherical radius R) the symmetric doubly-excited resonant states have well defined lambda , mu nodal structure indicating a near separability in prolate spheroidal coordinates. For fixed lambda , however, a clear mixing of R, mu nodes is demonstrated. This corresponds to a breakdown of the adiabatic approximation and can be understood in terms of the classical two-electron motion.