Magic numbers, ... and strings, all over the place [0]. Memory leak galore (debug code?) [1]. Probably buffer overflows all over the place, here's one I noticed [2]. I suspect others given the proliferation of opaque pointers and memcpy usage.
[0] https://github.com/skypeopensource/skypeopensource2/blob/mas...
[1] https://github.com/skypeopensource/skypeopensource2/blob/mas...
[2] https://github.com/skypeopensource/skypeopensource2/blob/mas...