According to the authors of Light Speed Variation From Gamma-ray Bursts[1], photons from gamma ray bursts show a speed dependency on their energy. Though inconsistent with current theories, the observations described in the paper are consistent with predictions of quantum-geometry dynamics and easily explained by it without violation of the speed of light.

[1] http://arxiv.org/pdf/1607.03203v1.pdf

Note: Presented here section excerpt from the 3rd edition of Introduction to Quantum-Geometry Dynamics

We show how the two constants of QGD can be derived from experiments and how from these we can convert its natural units in measurable units.

PDF file can be downloaded here