  We study corrections to tri-bimaximal (TBM) neutrino mixing from
renormalization group (RG) running and from Planck scale effects. We show that
while the RG effects are negligible in the standard model (SM), for
quasi-degenerate neutrinos and large $\tan\beta$ in the minimal supersymmetric
standard model (MSSM) all three mixing angles may change significantly. In both
these cases, the direction of the modification of $\theta_{12}$ is fixed, while
that of $\theta_{23}$ is determined by the neutrino mass ordering. The Planck
scale effects can also change $\theta_{12}$ up to a few degrees in either
direction for quasi-degenerate neutrinos. These effects may dominate over the
RG effects in the SM, and in the MSSM with small $\tan \beta$. The usual
constraints on neutrino masses, Majorana phases or $\tan \beta$ stemming from
RG running arguments can then be relaxed. We quantify the extent of Planck
effects on the mixing angles in terms of "mismatch phases" which break the
symmetries leading to TBM. In particular, we show that when the mismatch phases
vanish, the mixing angles are not affected in spite of the Planck scale
contribution. Similar statements may be made for $\mu$-$\tau$ symmetric mass
matrices.
