Cannot Join Multiplayer Server

Put the summary of the bug you're having here

What I expected to happen was...:
I join a multiplayer server, and I THOUGHT I would join like normal.

What actually happened was...:
Minecraft froze upon pressing the "Join Server" button, freezing at the "Connecting to server" message. After a while, it kicks me for this:

Failed to login: java.io.IOException: Server returned HTTP response code: 503 for URL: http://session.minecraft.net/game/joinserver.jsp?user=(username)&sessionID=token(random characters)&serverID=(random characters)

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. Join a server.
3. You will get kicked for the reason above.

• • • NOTICE: THIS OCCURRENCE IS TOTALLY RANDOM, AND WHEN IT HAPPENS, IT LASTS FOR A LONG WHILE. ***

Fixing this is very important because without the fix, it is impossible to join servers.