Hi,
the attached blend renders differently in the glare node on 2.63 vs 2.64-RC2 (also SVN51011), I've tried Linux and Mac. 2.63 works as expected, 2.64 not. The blend file provokes the cause. On 2.63, all RGB levels below 0 are obviously clamped before mixing, so the additional glare becomes fully visible ("lighten"). On 2.64 the effect seems to be just added to the negative values, the glare stays below black. BTW: Although clipping is enabled in the RGB-curves-node, it doesn't seem to help here.

Clamping in the mixing block is also not a real solution as it kills glare-producing highlights :(