Possibly just Java being unreasonable, but I get a horrendous crash whenever I try to log into a server where my character is in the Nether. I sometimes see the World for a few seconds before crashing, but most of the time it never leaves the log-in screen and just tells me it's stopped working. Or even entering the Nether on a server i'm already logged on to.
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Duplicate of MC-2749, please use the search function to see if your bug has already been submitted. Currently over 54% of tickets are being closed as duplicate.
Your Java is way outdated:
JRE version: 6.0_30-b12[Updating http://www.java.com/] to recent 1.7 has resolved the crashes of MC-2749.
Immediate crash