Tensorflow NaN bug?

I'm using TensorFlow and I modified the tutorial example to take my RGB images. The algorithm works flawlessly out of the box on the new image set, until suddenly (still converging, it's around 92% accuracy usually), it crashes with the error that ReluGrad received non-finite values. Debugging shows that nothing unusual happens with the numbers until very suddenly, for unknown reason, the error is

[p-baleine]

sigmoid nan