Lets say we have a wave which is linearly polarized and is incident to the surface of a imperfect conductor, which we will say is the plane $z=0$. Suppose the incident wave $E_i$ is parallel to the surface.
We know there will be a wave $E_r$ which is reflected, and because the conductor is imperfect, there will also be a transmitted wave $E_t$ which weakly penetrates weakly the conductor (skin effect).
To calculate reflection/transmission coefficients we need the boundary conditions. We have:
$E_i(z=0)+E_r(z=0)=E_t(z=0)$ because the electric field is continuos across the boundary in the direction tangentiel to the surface.
Assuming no surface courant, we should also have continuity of the magnetic field across the boundary: $B_i(z=0)+B_r(z=0)=B_t(z=0)$.
However, according to slide 10 of https://www2.ph.ed.ac.uk/~playfer/EMlect15.pdf, the second relation should actually be $B_i(z=0)-B_r(z=0)=B_t(z=0)$ (*).
The above link gives the correct answer for the relfection/transmission coefficients, so the equation (*) they give is not a typo.
I can't figure out for he life of me the source of the minus sign!