If you keep your Minecraft window open for a long time and then connect to a server, it says Failed to connect to server: Bad Login. Once you reopen Minecraft it will fix the issue.
MC-23963
If you keep your minecraft window open for a long period of time, it says Bad Login
Resolved
Invalid
0
(Unassigned)
Unconfirmed
badlogin, multiplayer, server
Minecraft 1.5.2, Minecraft 1.6.1, Minecraft 1.6.2
Retrieved
Is this still a concern in the current Minecraft version 1.7.2 / Launcher version 1.3.3 ? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.