13w48a you mean? I think Grum means that the issue is on twitch side...however streaming with Wirecast works perfectly fine so I doubt it's on twitch side but with Mojang own implementation.
Interesting thing: streaming in 1080p works (with Mojang implementation) but not in 720p on my side (but my upload speed can not handle 1080p streaming 😃).
13w47d: same problem
Yeah, sometimes one frame appear with ultra-low quality and freeze...
Well finally 1.7.3 pre-release fixed the issue for me!