Error (Help Needed) Premium Needed?

Okay, I never played MineCraft for quite a while now. Because my mom took my laptop away, and now I've got it back. I have being recieving error from MineCraft. The error is a javascript error.. and I used the Mineshafter client. It won't load. At all, and also when I used the Minecraft.exe and play offline. And join the server, it says Bad Login. What the hell? I mean is the server premium? I can see 20 people joined. Don't tell me they got premium? I can't use mineshafter. I haven't tried MineShafter Squared, I won't until I need to. I have to redownload if needed to.

Can you tell me why it says Bad Login? Please reply.

its not premuin its just that the server has lag issues so u might nid to login continously till u finally get in