I'm playing 0.45b with world generetion modified by myself, and I've received the following error message after playing for 2-3 hours (it happened 2 times only, and I'm playing this map since about 36h ago (total in-game: about 10 hours).
It seems one guy had the same problem, Plzach, and he posted one topic on General Discussion. So I've decided that the best would have the topic here.
I'm trying one thing that Bryan told me to do: Trying to downgrade my Java. I had installed Java 6 up 31 - in 32 and 64 bits. And also 7 in 64 bits. (newest current update is 32 for java 6). So I'm installing right now Java 6 up 30 - in 32 and 64 bits (my windows is 64 and my browser is 32 bits).
I'll come here and tell you if it worked (like I've said - it might take a wile, because it only gave the crash after 2-3 hours.