As I found out yesterday, the girl who voices Sweetie Belle has her own Youtube channel, and she does a Let's Play of Minecraft. Well... apparently Sweetie Belle freaks out when Minecraft quits on her:
http://www.youtube.com/watch?v=HIqikVRz2rs
Listen in particular to parts like 9:23-9:32, she sounds exactly like Sweetie Belle! The angry letter she writes to Minecraft had me in tears from laughter, and the ending... the fact that she slams pillows angrily and shouts "SWEETIE BELLE RAGE!" is both shocking and amusing.
And here's a small animation someone did of parts of it:
http://www.youtube.com/watch?v=Z1Z-m5lbqjE