The command /whitelist add suggests player names in lowercase only. This has no negative effect but can be pretty irritating.
- Use the following command to load a player with a upper case letter into the user cache
and deop him afterwards
- Type the following and press the TAB key
→ The server completes "notch"
The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.
The reason why this happens is because the field net.minecraft.server.management.PlayerProfileCache.usernameToProfileEntryMap stores usernames in lowercase only.