You might have received a "Failed to login: Bad login" error when trying to connect to a server on some older modpacks. This is because Mojang has disabled their old authentication servers (login.minecraft.net), which means that Minecraft versions older than 1.6 can't authenticate anymore. For example: Tekkit Classic.
If you're running a Minecraft server for these versions, the only workaround right now is to set your server to offline mode (
online-mode=false) via the server.properties file located in the root of the server folder, which will disable all authentication, so it's highly recommended to install a plugin like AuthMe (https://dev.bukkit.org/projects/authme-reloaded) so your players are at least acceptably safe.
This issue is listed on Mojang's bug tracker at https://bugs.mojang.com/browse/WEB-1429.