If you are recieving this message
Internal Client error: session.minecraft.net
Then the problem is not with your Minecraft client, nor the minecraft server. It's your DNS server.
Try doing this:
Start from your root directory (usually drive C)
then go to windows -> System32 -> drivers -> etc
open the "hosts" file using notepad read the paragraph to help you understand the file then enter this on one line by its self:
184.73.223.40 session.minecraft.net
save and close.
That should work.
If you're afraid of editing any files in your system32 folder then don't do it. It's up to you.if you want someone to confirm this fix, go to this post.
http://www.minecraftforum.net/topic/1328904-internal-client-error-sessionminecraftnet/page__st__20#entry16268374
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