PBRT path tracing: negation of ray direction before next event estimation
The incoming direction $\omega_o$ (coming from eye) is flipped in pbrt:

"Another convention we will follow is that the incident light direction $\omega_i$ and the outgoing viewing direction $\omega_o$ will both be normalized and outward facing after being transformed into the local coordinate system at the surface. By convention, the surface normal always points to the "outside" of the object...".